First of all, thank you for putting your time and effort in Sigil. I've only just started with it but it looks promising. The only real issue being that it's rather slow, but I understand that's mainly due to Webkit and you've already found a solution for that so hooray
I'm really happy with the search and replace, which wasn't there before (or I've been blind), especially because it supports regular expressions. I'm having some trouble with them tough, but that seems to be because different flavours of regexps. I run into that with Calibre all the time, too. Anyway, I'll be trying some other regexp testers than the one I currently use because I'm beginning to suspect it differs from most other libraries in its interpretation of the expressions.
Something I'm pretty sure there is an issue with is sudden changes of text size. As you can see in the attached screenshot, the text size between to chapters differs though under the hood I can't see anything different. Sorry if this has been brought up before, I couldn't find it on the forums.
This is a plain text file that I imported and converted to LRF using Calibre. Since I don't have access to the original text file on this computer, I used Calibre to convert the LRF to HTML which I opend with Sigil.
Edit: Something else, when working in the split view, Sigil often crashes when switching between them.
Btw, I'm running Vista x64 on an AMD Phenom quadcore something with 3gigs of ram.