For a wide selection of sounds from realistic acoustic instruments to electronic ones, don't look any further.
The Pianos and Organs are inspiring - B3s, Rhodes, Wurlys are all excellent. The Strings are very good. There is an amazing collection of Synth sounds. You may get more flexibility with other VSTi's when it comes to creating sounds, but you can drop these into a song and get on with actually writing and they sound fantastic.
That is probably the greatest thing about SS - with very few exceptions, you can find the sound you want and it fits in the mix perfectly with a minimum of hassle, a bit of EQ and reverb. I prefer the effects in Logic to their equivalents in Sampletank, but these are still of good quality.
There is a lot of variety in the drums and some great kits, including some I-Map kits. Unfortunately I have no idea what an I-Map kit is, and the manual doesn't help.
I wasn't quite so impressed with the Brass/Wind section - I didn't think these were up to the same standard as the sounds elsewhere, but there is still some great stuff here!
While I personally won't use them so often, there are some great sounds in the Chromatic and Ethnic sections. Guitars and Basses are fine, although being a guitarist myself, I won't be using them (to be perfectly honest, I've never found guitar sounds I'm 100% happy with in any product).
It is quick and easy to get up and running (so long as the registration process goes smoothly), but I would have liked some description of the sounds in the manual to make finding the right one that bit quicker - some of the names are pretty cryptic and there is so much on offer.
I would also like an upgrade path to get the extra MIDI inputs, AKAI import and outputs - as it is, you have to buy the whole Sampletank XL or L packages complete with sounds.
The customer service is some of the best I've ever experienced, not that I've had any problems.
To sum up, the best value, most inspiring VSTi I've found.