Finally, today I dumped my WordPress installation and switched to b2evolution. Playing around with b2evolution for the last few days, I am surprised how such a nice blog software could exist without me noticing it earlier.
Unlike WordPress, b2evolution has cleanly written structured code and lots of more features (and you don't get new security issues every week or so). Sure, there aren't as many plugins and themes available yet, but skins and plugins are well structured and easy to learn, so it was no problem at all to get all the features.
Migration was pretty easy. I just installed an export plugin to WordPress that exported all postings into MovableType format and imported that data with the b2evolution MovableType import function. All postings, comments, trackbacks and categories were converted and I later added tags to each posting manually.