Well, I just got it a few days ago. I've tried all the channels and the pres and channels sound great. I just ran an 8-track mix off my hard disk and experimented with the busses and "mixing by hand" which I haven't done in years... I was pretty impressed with the sound and flexibility. What I like the most is that it's pretty damn clean sounding, but not sterile, it's extremely flexible, and the eq sounds awesome. It's very British, sort of that warm but not overpoweringly warm, clean but not clinical sound. I also got the meterbridge, so I'm really happy with the metering too.
Honestly, the biggest thing I don't like about it is the size. It's fairly big and heavy. With the meterbridge, it's 39" wide x 29" deep x 11 1/2 " high, and it weighs around 70 pounds. If my "studio" was more than a garage, this wouldn't be an issue. And in comparison, the Mackie 24/8 is within a couple inches of any of these measurements. 24x8 mixers are just big!
I also have minor issues with the manual. I like the fact that they publish every single schematic for the entire mixer in the manual (very helpful for any electronic repairs you might want to do), but I think it's a little bit short on usable content. However, I'm often picky about stuff like that since I write manuals for a living. I may end up writing up a little online user guide for it...
There are a couple of other Soundtracs Topaz users here (SpongeBob, Downside Studio, MISTERQCUE, richthirst, Folkcafe) who have used it for more projects, and can probably give you their ideas of what they like/dislike.