I've had Sonic Synth for about two months now and I think I come to appreciate it a little more every time I use it. I wasn't overly impressed with it when I first got it, and I still have to admit it's not my "dream module", but it has become one of my most used VSTi's.
I agree with pretty much everything that's been said in the previous reviews. You get a bit of everything here. It's like the swiss army knife of VSTi's. If you can't find a lot of sounds that are usable to you then I don't know what you're playing, but it probably isn't very musical or it's loop based.
Even if you already have a VST sampler like EXS-24 or HALion you probably don't have any where near as comprehensive a collection of sounds as you get with Sonic Synth (definitely not with EXS!). I use SS along with EXS. This way I get a big selection of general and regularly used sounds like pianos and drums, and then I can have EXS for the more specific or esoteric sounds that I'm after that SS doesn't cover, or doesn't cover as well as a sample cd dedicated to one instrument or type of instruments would.
As for the Sample Tank engine, imo it's kinda ugly and utilitarian, but it's simple and effective, no manula needed. Selectable skins, a less tedious authorization process and a few things like portamento, assignable ADSR, and some basic editing features would be greatly welcome. The built in effects are very good though and let you manipulate the presets quite a bit. I change the effects on nearly preset to my taste, particulary the eq.
For those that question the expense of Sonic Synth consider this. Probably the cheapest hardware "romplers" you can buy today is the roland JV-1010. that costs about $400 and comes with 16 megs of sample ROM. Sonic Synth on the other hand is $250 and has the equivalent of over 2 Gigs of sample ROM. If you could buy 2 Gig's of sample ROM for a JV-1010 you'd have to buy 250 expansion boards at $225 a pop, that's $56,250! You be the judge.