Someone posted this poll on reddit, and I thought the results were interesting. It only lists the big guys, but KDE4 is far and away the most popular listed. I didn't realize it was that widely used. I would have expected a better showing from the other desktops.
http://pollator.com/polls/which-linux-desktop-environment-are-you-using
IMO, of the more common ones, XFCE > LXDE > KDE4.4+ > * > Gnome.
XFCE has nice applets, and a container for Gnome applets, so there is a lot more you can cram into a panel than LXDE.
LXDE is awfully minimal, but works very well. IMO, XFCE and LDXE are best used with full installs of Gnome and KDE on the machine, so you can configure the behavior of Gnome- and KDE-dependent applications with relative ease. Either LDXE or XFCE are quite nice with Openbox, too.
Unity might be OK with touch, but with KB/mouse (or trackpad, or titmouse), it plain sucks.
Gnome is always frustrating to use, and Gnome applications tend to lack useful features (where is the test integrity button in File Roller? That was my most recent, "ugh, Gnome!" moment, after transferring some backup tarballs over an unstable connection).
E17 can be nice, but it takes a lot of tweaking for good UI efficacy on a halfway normal computer (I'd like to try it on a space-constrained touch system one of these days, to see if the defaults work out better with that kind of situation).
KDE is good, but busy, kind of a pig, and configuration tends to be non-intuitive. Little things, like how to make shortcuts on the panel, how to move things, how to get or get rid of those folder box gizmos, etc.. However, it has a lot going for it, including system-wide configuration options (instead of just WM/panel-related), and acts close enough to the way Windows 7 does, for anybody used to Vista or 7 to be able to just sit down and start using it.
I notice a comment mentions RazorQt. I haven't tried that one. I aught to make myself a reminder about it.