I just installed Gnome 2.6 out of Debian experimental, and I'm pleased by several things.
It's faster. GTK seems to be faster--menu redraws are quick, and the whole interface feels pretty good. Most impressive, though, is Nautilus, which has gone from dog slow to downright snappy.
It reminds me of the Mac. I'm an old Mac user, so I'm delighted by the "spatial" version of Nautilus. Essentially, there's a one-to-one mapping between directories and folder windows, and folder windows have persistent properties, including location. This was a really robust UI design, and I'm glad to see Nautilus borrowing it. I just wish they'd simplify the screen even further, and add support for those nice Finder triangles.
All in all, the UI situation is very good in Gnome 2.6. I'm impressed at how quickly the Linux desktop is becoming truly usable.