Not bad...not bad at all. It's a bit slow but on a Mac Mini everything seems a bit slow. That said, the features match up pretty good and the compatibility to MS Office is dead on. It doesn't contain all the features of Word but those omitted are often neglected as bloatware anyway. 90% of most users will find Word and Excel features within ThinkFree Office. Show does an okay job of PowerPoint 2004, although it does not contain some of the more recent image & transition enhancements; that's to be expected considering that even the Windows PowerPoint does not contain these.
The lack of customization for the Toolbars while expected of a cross-platform Java-based program is still a disappointment. It does not let you combine separate bars onto one line which is necessary for mobile users with wider than higher screen space. As I said, speed is a bit slow but really only in startup; an option allows you to reserve the launch memory for quicker future access if you're willing to part with it. Otherwise, actual usage is steady. Once again, Show is where I saw the slowest responses; especially concerning Slideshow presentations. Transitions tax the CPU heavily.
That said, at $50/seat, it's a great alternative to Office; it even breaks even with the MS Office Student/Teacher version. If Apple does not come out with its version of Excel for iWork soon, I may jump in to ThinkFree Office.