I was sorely tempted to give SampleTank FREE a 10 rating just because it's so incredible that anything this good could actually be free. My first experience with the IK Multimedia's products came when I bought Miroslav Philharmonik, whose symphonic instruments exceeded my expectations in terms of sample quality, ease of use, and overall usability in my mixes. Thus encouraged, I figured I had nothing to lose by trying SampleTank FREE. I was right.
I can't say I like all the instruments, but that's true of every virtual instrument I own, so that's hardly a negative. Actually, I'm impressed with most of the instruments. Right out of the gate, I found great drum kit and bass sounds that were immediately useful in my compositions with little or no tweaking. The electric pianos are just OK, but maybe there are better instruments in the full version. I was pleasantly surprised by one of the vocal instruments (Soprano Ahh), which is remarkably realistic for the right kind of part. In fact, I'd say that the key to using SampleTank (at least the FREE version) is knowing when and where to use the instruments for their strengths.
I was dismayed to find that using multiple instances of the SampleTank plugin for certain instruments produces a horrid crackling kind of distortion (in Logic Pro 9.x, at least). I called the bug to the attention of support, and their responsiveness was most impressive. They expect to fix it in the v3 release.
Overall, I'm sufficiently impressed with SampleTank FREE to buy the full version, SampleTank XL, but I'll probably wait until the v3 release.