I've been running UseModWiki since April 2001. I've tweaked it a bit, of course, to match the needs of the sites I run. All the functionality changes can be found at ftp://ftp.gw.com/pub/people/kim/patches/usemod/ in UniDiff? format.
Concerned about performance I started looking at OtherWikis that might run faster on my systems. I found that I was already fond of many features of UseModWiki, so switching from it would not be an easy decision. Eventually I got to WikkiTikkiTavi, which immediately gives you the look and feel of UseMod. Some of the TextFormattingRules are different, though.
I recently upgraded the web server to much more powerful hardware, and saw an improvement in the speed of the wiki. But you still had to wait a few seconds for each page. Then I read about PersistentCGI and decided to try out something like that. I installed mod_perl and now pages just fly out of the server!
I do have WikkiTikkiTavi installed also. I like the idea of having all the pages available in a MySQL? database. I also like the templates (and should look at WikiPatches/Templates carefully). WikkiTikkiTavi is certainly running fast, although it is not a fair comparison in my case, since I run UseMod on older and slower hardware.
So UseMod is definitely staying here now, thanks to mod_perl. But I like having WikkiTikkiTavi around, too, for good comparison. I hope to bring the two closer to each other, at least as far as TextFormattingRules go.