I love Opera, but is spoiled by a few small problems with some sites.
Most of the time this is down to pages being designed badly or using non-standard HTML.
Some of the time I suspect it is my fault for not having the time to understand Opera properly.
The bottom line for me is not so much whether the 'bells and whistles' work, but how quickly and easily I can do my work.
The downside of IE5 and IE6 for me is the way it handles favorites. Its clunky and feels bad compared to Opera and Mozilla/Netscape recent builds.
The downside of Opera (at least with my configuration) is that if I type in a bad URL it does not retain the bad URL for me to edit - just junks it and makes me retype the whole thing, I find this irritates me 3-4 times a day, (yeah, I know, I should type better)
As a more serious problem, neither N6.1/Mozilla or Opera will work with Positiontech (for my Inktomi registrations) or with my Plesk server control panel. This seems to point at the Java module, which I believe they share.
Bottom line - I use all three, but would prefer to use Opera more.