I used Flock for over half a year, after blanky told me about it.. I guess it's just the skin that gets me. Anyway, a peculiar thing happened the other morning, after having it installed for 3 months (since my format) and visiting just over 23,000 web pages without clearing my history/cache, it screwed up. As you'd expect it got slow, but it also switched an image on Wikipedia with a random image from my cache... and after that I just thought, "That's not reliable at all!" and I'm now using Firefox. It's good for a while, but the novelty seems to wear off, especially when it starts screwing with your stuff.
The only feature of Flock I really enjoyed was the web snippets and live bookmarks, and there are extensions for Firefox to allow live bookmarks, so it's all good.
I'll give it a go when the new version comes out, but I don't feel I'll be returning any time soon.