When I purchased my last machine SSD's  were just becoming available however I they were quite expensive so I did nt bother...
 
This time round I am considering one but I'm unsure what size to get, also I'm wondering if having such a comparatively small drive causes problems when using Sonar combined with multiple VST instruments when they are installed on a different drive.
 
Also does an SSD lead to a 'Siege' mentality and constant worry about remaining drive space ??

What has worked well for most is a 3 HD setup:
 
C Drive: OS and all software (a 250GB SSD works really well)
D Drive: Sonar Projects (a 1TB (or your size choice) 7200 RPM HD)  
E Drive: Samples  (biggest HD (3TB or more??) depending on how big your sample library collection is)
 
If you can afford it and need  it, a huge SSD samples drive will make loading sounds a breeze.
 
Of course, the above numbers are all subjective to your needs.......and that configuration has worked out well for many of us. 

Agree with Mesh.
If you go the pure SDD route you don't need to buy seperate SDD's just partition them accordingly. The only advantage with separate partitions is keeping your files in a nice structure.
 
If you can afford a 1tb SDD (or even 2tb) and bought an identical second you could mirror them in case one fails.
Check out the Samsung EVO range. Good 5 year warranty and reasonably priced.
Most important is backing up to an external hard drive with something like acronis true image.

Depends on how big the SSD is. If you get a large enough one then no, no issues about hoarding space. Its only an issue if you purchase a drive that is too small for you needs. And of course as Mesh noted you can (and probably should) mix SSDs and HDDs.
 
SSDs are good for things you want fast load times on like programs and samples. HDDs are good for bulk storage.
 
Personally I have an SSD for OS and apps, another for samples, and then two HDDs one for data, one for backup.
 
Basically make a list of what you want on an SSD and figure out how big it is. Add in at least 25% more for other uses and then see what you need. See if you can afford a drive of that size. You probably can, SSDs are fairly economical these days. Like you can get a Samsung 850 EVO 500GB for $180 on Newegg. That's a nice drive that'll hold a lot of samples right there.
 
Also with magnetic drives, Seagate makes something of a compromise in their SSHD line. They are magnetic drives with a bit (like 8GB) of flash on them. They use that to buffer reads and writes. It means that they handle fast multiple access more gracefully than HDDs. Still nowhere near as good as a real SSD, but if you get one for you data drive it can be a nice compromise between cost and storage.

IfItMovesFunkIt
When I purchased my last machine SSD's  were just becoming available however I they were quite expensive so I did nt bother...
 
This time round I am considering one but I'm unsure what size to get, also I'm wondering if having such a comparatively small drive causes problems when using Sonar combined with multiple VST instruments when they are installed on a different drive.
 
Also does an SSD lead to a 'Siege' mentality and constant worry about remaining drive space ??

As was mentioned, if you're using large sample libraries, you want a three drive configuration

• Boot drive
• Audio drive
• Samples drive

SSD as Boot drive is nice.  The machine boots faster... and is a bit "snappier" (apps open quicker/etc).
 
For the Audio drive, there's no sense in going SSD.  A fast conventional HD can sustain 100 solid 24Bit/44.1k tracks.
 
Where SSD really shines is for running disk-streaming sample libraries.  A fast conventional HD sustains 150-190MB/Sec.  A fast SSD sustains over 500MB/Sec (roughly three times the speed - which means three times the polyphony).  If you're doing huge orchestral mock-ups (pulling 1500 notes of disk-streaming polyphony from many simultaneous disk-streaming sample libraries), you'll need to use multiple SSDs.
If you're polyphony demands are smaller (say 128 voices), a conventional HD works fine for streaming samples.
 
Note:
Even if you're using RAM based sample libraries (like Superior Drummer 2), you don't want 60GB of samples loaded on your Boot drive.  That makes backup slow/tedious... and consumes lots of (redundant/unnecessary) space.
 

As for HDDs I prefer drives with less platters.  The largest I have is a 2TB. 1 OS, 3 storage.  I have not filled them up to where any of them have used 75% of their capacity.  I have loops, Komplete, Kontakt, EW, VSL, Live Suite, Refills.
I have good luck with WD Blues. 1 TB are under $60. 

With 1 TB fast mechanical drives selling for around $40 I find it difficult to see why I would want to pay $100 for a system drive that holds a quarter as much just to be able to boot the machine a little faster. After bootup, machines with a reasonable amount of memory are running the system from the loaded memory image, so system drive access speed is not something you are likely to notice. Of course I started when the fastest drive was a 5 1/4 floppy, so I probably have learned more patience than some people here. No argument if you are loading samples on real time demand, but many romplers actually run from memory or have caching that pulls them into memory on first use, so that the SSD benefit for those applications is negligible. I tend to partition drive 0 into a relatively small system partition that is small and fast to image, and use the rest to copy important data from a primary data drive so that I have two copies on different drives on the same machine. That does not provide the same security as cloud or removable backup, but it is fast, easy to implement, and much safer than a single copy against drive failure.

It isn't so much for the boot time, but the application load time, and general system responsiveness. When you have an SSD bogging down your system is hard to impossible to do. Even if multiple things are launched at once it doesn't impact your ability to use it because they service simultaneous requests much better. Likewise programs load lightning fast. I play a lot of video games and it is great not having to wait on loading screens, stuff just comes up nearly instantly.
 
Also the thing is you often have no need for the storage capacity of a magnetic drive for a boot/apps drive. 1TB is more than most people would use for that purpose. So comparing a 500GB SSD to a 1TB HDD actually makes sense since the SSD is plenty large enough. As a simple example I have a 512GB system SSD and I have only about 200GB used. On it I have my OS, Sonar, Vegas, Soundforge, associated audio plugins, Office, browsers, editors, and a number of games including Dragon Age Inquisition, Final Fantasy 13, and Shadowrun Hong Kong. I have a fair bit of stuff, and yet it all fits nicely on an SSD, and would fit fine on a 250GB SSD in fact.
 
So ya, you can get magnetic drives for less, nobody questions that, and for bulk storage SSDs are way too expensive. But OS and apps work pretty nice on a smaller SSD and then your system is just extremely zippy.

+1 to the above, 250GB as an OS/programs drive is fine with data on other drives. Multiple drives is actually faster as you can get data to/from them quicker. SSDs excel at massive read operations since they are not limited to one file at a time with seek time added between as the head assembly moves.

Heads in a HDD are physically linked, so even partitioned only one file can be read/written at a time (for all partitions).

Sycraft
 .....and a number of games including Dragon Age Inquisition, Final Fantasy 13, and Shadowrun Hong Kong. I have a fair bit of stuff, and yet it all fits nicely on an SSD, and would fit fine on a 250GB SSD in fact.
 

(Sorry OP for the slight tangent)
 
Sycraft,
As far as games are concerned, I only have Steam & Origin running on my SSD drive. All my game data is on my samples/data  3TB drive. Did you install these games on the SSD drive and are the download content/updates stuff on another drive? or do you have all these games and associated data on your SSD?
 
I avoided putting all the game data on the SSD as it's only a 250GB drive and I know these games can take a HUGE amount of space. (I mostly play on Steam and the games do load very quickly......just a few secs).