The latest beta (2.5b22) attempts to address some of the issues users have been running into when upgrading. It fixes various conflicts with various configurations and tries to be give the upgrader as little work as possible. I will be very interested to hear whether the attempt has worked! If you run into problems (new or ones or old) please use the bug report screen under settings | similar posts to let me know.
One issue that I have addressed concerned indexing the blog. In an attempt to simplify matters I have made the initial indexing on first activation automatic and I have brought the index management under the settings subpages as some users found the separate Manage page complicated things.
I have had the occasional report of Similar Posts not working when the WP-Cache plugin is installed. I’d like to hear more, either way, if you try to run both. I can confirm that Similar Posts is working fine alongside WP-SuperCache on my development site.
Anyway… as the fireworks say ‘light the blue touchpaper and retire’!