Last time I tried Viewer 2, I couldn't have a user profile up next to a group profile, couldn't open several group profiles at the same time, couldn't easily open an inventory to drag an item into a group notice creation dialog (but I later learned that shift-ctrl-I might do it), and some things like that. I haven't tried it in a bit now, and I've seen from the change log that at least some of these things are fixed. But the "only one thing open at a time, and it's stuck in the sidebar" thing made it really hard to work with Viewer 2 for me.
There were also lots of weirdness with the search window, searches resetting when reopening the window after a teleport. Lots of the other dialogs lost their state when going back and forth between them too, like digging down into a user's groups, going to another user's profile from that group, and then not figuring out how to manage to find the first user's profile again. Ironically, the sidebar seemingly being modeled after a web browser, it did manage to make things a lot harder to browse, that way. Modern web browsers also have tabs and new windows to help with doing more than one thing at once, but the SL sidebar was very single-minded/one-tracked when I tried it.
Another thing I really hated was getting people's RL info right in the face (with RL pics!) when checking their profiles. As a role player, that really messes with immersion. RL info contrasted with RP info = really bad.
I liked the tiny notification icons, one for each IM window, though. That helped me be able to ignore group IMs when I was focused on other things, while still seeing one-to-one IMs.
So yeah, I had many reasons to feel that Viewer 2 wasn't for me (the dark GUI not being one of them). But I have good hope that they will eventually manage to fix most of those reasons. Eventually I might want to switch.