We just switched to Skype from IBM Sametime.
It seems better for the most part (wouldn't be hard), but the UI is confusing and very, extremely, astonishingly, impressively slow. I can type a whole sentence, push enter, and alt-tab away before anything updates on the screen. Same thing with highlighting- Click, drag, release, wait, contemplate life choices, check personal phone, maybe grab a bite to eat and then it'll show the highlighting and I can control-C it.
I'm also not a fan of the whole "everything is giant and takes up half the screen for no reason" thing, either.