My Qt5 12.04 Sigil seems to function so far. I checked it with "TheNewGenetics.epub" ( free from
http://publications.nigms.nih.gov/order/index.htm), which Sigil tag 0.62 Qt4 was not able to load - actually this prompted me to try next Sigil git issue - and it loaded fine.
I'm not sure I like new behaviour of F10 (Preview Window), personally I'd prefer the old way - maybe because of my 20 inch monitor.