I've tried it on win 98 and Linux. It is much better than netscape 6. It still isn't perfect, but a lot of bugs and annoyance seem to be gone. SSL works -- I am told that this is the first release that it works for Macs. It also seems faster than netscape/6. One thing that drove me nuts with ns6, but is fixed on this mozilla release is that cutting and pasting from the source code now works without stripping out the newline characters.
I believe wheelmouse support is complete... Speed has improved, but it depends a lot on memory, CPU speed, and OS.
Win9x/ME is far, far slower than NT/2000. In some cases, I've heard that it causes memory thrashing on 96MB machines on win98. Anything slower than 400MHz is also likely to have a problem.
Don't bother with Mozilla 0.7, though... Mozilla 0.8 branched on Wednesday, I believe, and NS6.01 should also be newer than 0.7. I don't know at what point they branched, though, since NS isn't nearly as open about things as mozilla.org. 0.8 should be available for download within a few days.
The really big speed boosts will likely come with Mozilla 0.9. The image library contributes mightily to both loading times and runtime performance problems. A new one should land any day now.