Question on Sampletank - what is compelling?

If you have SampleTank 2XL or any of the other ST Instruments
(SampleMoog, Miroslav, SonicSynth, SampleTron) and want a better
64 bit interface....then that's the only way you'll get it I'm afraid (right now)
But, is there anything that's going to be better then what you have with
Kontakt or Omnisphere?  Not really...not even in the same ballpark IMO
Sample-wise or otherwise....

But, most believe I'm biased, so take my opinion for what it's worth.

ST 3 does offer a lot of high quality sounds IMO and comes outfitted with beau coup excellent effects on board.  But if you have Kontakt and the other instruments you named, do you need another sample player?  That I cannot answer.  But I support what Larry said - if you have Sample Moog and Sonic Synth, which obviously you don't or you'd be talking about the upgrade price, you would want ST 3.  I just went through a bunch of my Sample Moog sounds and it's "welcome back, Sample Moog!!!".  I'm very please to have ST 3 on my system.

I concur with several points made by previous posters. I have not yet received my box set.  However, the reasons I went ahead and ordered it were these:
1.  Its 64 bit
2.  It has a larger and finally readable  GUI
3.  It integrates several IKM effects from T-Racks right into ST3.
4.  It can import the other IKM sound libraries (Miraslav, SonicSynth, SampleMoog, SampleTron); I own all of them.
 
To be honest, I have not used ST2.5 in years--primarily because it is just too small to see; I have very bad vision.  I actually like the sounds found in many of the previous IKM synth libraries, but have not used them because of the GUI.  So, I went ahead and purchased ST3 not to get its new sounds so much as to regain access to all of my IKM sound libraries in a usable, readable, 64bit native application.  From what I have read here on these forums, others are not overly impressed by the new ST3 library; I have so many specialty libraries for Kontakt that will probably surpass ST3's core library;  I can live with that (if what others are saying is true), BUT I look forward to finally getting to use all of my IKM toys (libraries) once again in the new GUI.  Hopefully, having this nice GUI with improved visual access with spark some new creativity in me; it will be like getting all the IKM libraires at once (almost like the first time) again.  For $99 (which is what it cost me last month), this is actually an affordable price to pay for me to finally gain visual access to all of the IKM catalog.  
 

Thanks for the input so far.  Not exactly a resounding sentiment that at a $200 price for a first time buy it's a screaming deal and there's just too much to pass up.
 
I'm both overwhelmed and extremely satisfied with the ones I have, but there's always been a key reason for each the prior purchases because of something they offered that really made it worth it to make the jump.  In this case, I'm presently on the sitting on the sidelines side of the fence, but observing with interest.

I bought st3 upgrade for $99 at the presale. I am very happy with the purchase because
1. 64 bit sits with Sonar X3 64 bit nicely
2. tons of new sounds to compliment my komplete 9. There are some sounds that ST3 has but not K9. For Ex, many useful basses and gtr, jazz/pop brass, certain yamaha E Grand, Chinese Gu Zhen, different variety of synths and electronic drum(actually I prefered the synth samples in ST3 over Kontakt 5) 
3. Ease of use, you can quickly put together a track.
4. lots of useful and good sounding effects 
5. Bigger and Better GUI, ex, all 16 tracks in one page, all synth parameter in one page,
6. lots of useful midi drum patterns
 
I am saying ST3 compliments the Komplete 9, not better by any means,but just just different
But for now, I am using the vst2 version. The vst3 has some stuck note issue when using in PRV mode.

I have to echo cecelius2 - I have all the older libraries so $99 to get 64bit and a usable GUI was the trigger for me.

It's good to see the thoughts expressed here.   I'm starting to amble slightly toward the fence from my sitting entirely on the sidelines.
 
First, I have Amplitube, and of the several amp sims I have, I'm finding Amplitube to be a great experience for guitar sounds.  I recognize Sampletank isn't in the same category, but I'm thinking IK Multimedia has probably done some cool things in Sampletank 3.
 
Second, I'm starting to get intrigued enough to start looking at some of the demos, and of course that's quicksand.
 
Fortunately I have no cash, so it probably won't happen.  Sometimes the draw becomes very apparent and rationalization sets in as to why it may be okay to rob Peter to pay Paul because of a limited time and substantial discount.   I'm in the early stages of assessing my appetite to rationalize when I have no funds.

since this morning I am starting to warm up more to the faders on the mixer.  They are actually smoother than they initially seem