Serum is one powerful instrument. Look at the first minute or so after the intro to this video.
 
https://www.youtube.com/watch?v=TYhnSuuVqBE
 
Listen to how he sweeps the wavetable.  That effect alone for me would be worth the price of admission.  Not sure Iris2 can sweep wavetables like that under your control the same way.  They are very different.  They do different things.
 
Synthmaster is an amazing synth and it does cost less. And it is around $120. After some finding around I was able to sweep the wavetable the same way as per Serum.  Synthmaster has some incredible sounds and you can import your own sounds and raw materials too.  Next year it is going to be re designed and seriously upgraded to include many more synth engines.  Getting into it now will make it cheaper later to upgrade.  Synthmaster One is also another new addition to that family and it sounds very nice as well. 
 
Another synth not mentioned yet is Alchemy.  This is one amazing instrument.  Hard now if you don't already own the PC version which I do and it all still works fine.  I have many libraries for it.  Organic pads are one of its specialities. For new owners it is into Logic for you but I believe it is been advanced already.

Funny how things go.. this afternoon I've been checking out Synthmaster already, because it supports custom waveforms.
 
I already own SynthmasterCM and now tried the Demo of the real thing, plus watched some videos about SM and SMOne. But honestly none of these I do really like. Seems like the creator had a more technical than creative approach on Synthmaster, at some point realized this by himself and tried to overcome with the creation of SMOne - but wasn't really successful in my eyes. Most videos about SM / SMOne show that, too. E.g. the CMMagazine Video about SMOne barely manages to produce anything but a flat base sound in more than 20 minutes, clueless about what knobs to turn.
Some big names like Armin Van Buren suggest it's a great and powerful tool. If it is, it just takes too much time for a hobbyist like me. 
 
You forgot Tone2 Icarus. But as it almost the same price as Serum, it's pointless again, cause Serum has become a standard. Even if it's as good as Serum it would have to cost significantly less to compensate that. Hard times for being a VSTi Dev I guess.
 
Guess I'm gonna wait for a UVI Falcon sale or get it for next xmas or whatever.

Worth noting that Serum is available on a rent-to-own basis - $9.99/month until it's paid off. 

sharke
Worth noting that Serum is available on a rent-to-own basis - $9.99/month until it's paid off. 

Thanks, but in my case that doesn't change anything. It's not that I couldn't pay the amount, it's more that I don't want to pay that much for Serum. I guess most people around here are more serious about making music than I am. It's a hobby, just for fun and relaxation. (Which btw. I think is the best approach for good results anyway).

occide

sharke
Worth noting that Serum is available on a rent-to-own basis - $9.99/month until it's paid off. 

Thanks, but in my case that doesn't change anything. It's not that I couldn't pay the amount, it's more that I don't want to pay that much for Serum. I guess most people around here are more serious about making music than I am. It's a hobby, just for fun and relaxation. (Which btw. I think is the best approach for good results anyway).

You have plenty of tools already.  There is no such thing as the "one" synth.  Learn what you have, take it to the limits, then if you feel you need more you will feel better about spending more money.

abacab
 
You have plenty of tools already.  There is no such thing as the "one" synth.  Learn what you have, take it to the limits, then if you feel you need more you will feel better about spending more money.

 
This! I got Synthmaster a while back and at first I was a little put off by the learning curve and it is a slightly unusual layout too.  But once I really started working with it then I found it has a very impressive sound.  A deep complex ambient pad type track a la Eno for example could easily be constructed from nothing but Synthmaster alone.  Especially if you start importing complex oscillator waveforms.   All the elements are right there in front of you. (.....take it to the limits)  You may need several instances of Synthmaster in order to create a complex soundscape like this.
 
I am running an iPad in an Alesis IO Dock as an extra synth and it performs well in that mode.  I have got two Korg synths on there, MS20 and Polysix.   And a third.  Synthmaster for iPad.  I can't believe how fat and powerful Synthmaster actually sounds compared to the other two.  He has designed it for sound.  First and foremost.
 
Synthmaster One is a slightly different beast and some of the sounds are really beautiful.  Good companion to Synthmaster as well.  Sometime in the future I believe Synthmaster will take a major step upward.
 
I bought a great video from Groove 3 on Synthmaster.  After watching that a few times it really changed things for me.  It explains every single parameter.  Some real help like this speeds things up and opens things up big time.
 
It is not only modern synths that can do this.  I have got a flagship EMU Emulator hardware synth that I have picked up some amazing sounds for recently.  Third party patches of deep complex and ambient sounding pads.  The first patch has all the elements in it but the rest of the patches contain just one single element of the total picture.  Which can be transposed over the entire octave range and all manner of editing and sound shaping and effects can be applied to each element alone.  Then you can put it into multitimbral mode and sequence 16 elements of the ambient soundscape at once.  There is no limit to something like this and the possibilities are infinite. 
 

Jeff Evans
 
This! I got Synthmaster a while back and at first I was a little put off by the learning curve and it is a slightly unusual layout too.  But once I really started working with it then I found it has a very impressive sound.  A deep complex ambient pad type track a la Eno for example could easily be constructed from nothing but Synthmaster alone.  Especially if you start importing complex oscillator waveforms.   All the elements are right there in front of you. (.....take it to the limits)  You may need several instances of Synthmaster in order to create a complex soundscape like this.

I grabbed the Synthmaster expansion sound packs by Nori Ubukata "Historic Synth Giants Volume 1-4" recently when they were on sale.  Wow!!!  Listen to the demos on Soundcloud and you'll see what I mean!  I have new respect for the capabilities of Synthmaster.  And the version 3 update due early next year should be very interesting!
 
Expansions:
http://www.synthmaster.com/purchase.aspx
 
Nori was one of the first official sound designers for Yamaha DX series. Later on, he worked for Yamaha and Korg as sound designer and also as developing adviser. He was involved in almost all of DX,SY and TG series, VL-1 and VP1 of Yamaha , Korg's T-series, O1/W, Wavestation and Prophecy. He published several optional voice programs for DX and SY series, Korg M1 and Ensoniq VFX/SD. In 2005, he started designing presets for Arturia products, being an official in-house sound designer and demo music composer. At the beginning of 2012, he became a freelance sound designer and musician. He mainly plays theremin with CV driven analog synthesizers and has released 3 albums so far in Japan.
 

I forgot to mention expansion packs for Synthmaster.  I have quite a few myself including the Nori patches.  I used to own a SY77/TG77 synth and that was a beast to program.  Some of the best and most beautiful complex ambient sounds I have heard in a while.  FM7/8 also not bad either.  The unison mode sounds very similar to the original TX816.  I have got a massive DX library and there are many amazing ambient patches and soundscape type sounds in that library too. I was reading about Montage 7 and they have greatly expanded the FM engine inside Montage now.  Sounds awesome apparently.  It has thew biggest and most complex FM engine now. 

I'll throw Rhino 2 and Crystal into the mix.  
 
Rhino supports 6 operator FM Synthesis, reads DX7 and similar sysex.  The oscillators can be from a wide variety of waveforms or wavetables, or from a simple additive generator.
 
Crystal is a classic freebie that has oscillators that can be granulated or FM modulated, plus can import samples, and has a unique FX section.

I have been playing around with the free Fracture and Hysteresis from Glitch Machines, with some interesting results just putting some rhythmic material through them. 
 
Starting with a basic dry stereo drum pattern from an entire kit in Addictive Drums.  It can be morphed until it no longer sounds like drums at all, and the rhythmic patterns seem to create some interesting ambient motion.  It might also be fun to break the kit down to individual audio outputs and try processing each piece with a different preset that works differently on each timbre and pitch.
 
Also, the free (until June 8th) Diffuse from Surreal Machines can also add some additional delay and reverb to open up the space a bit more.