So I played around a bit with different combinations-- E16+Gnome, E16+KDE, E16 by itself-- and I have to say I'm not impressed with how well it meshes with Gnome or KDE, but that seems to be because they seem to want too much control by default. I would probably have to put in a lot of work to get them looking nice and playing nice together. The thing I notice the most, though, is how much faster E16 is by itself than when combined with Gnome or KDE (KDE especially). This could also be due to being run in conjunction with Gnome/KDE, but E16 definitely seems to run programs faster by itself than they do by themselves. Which is good, since that's one of the things the developers strive for-- a faster, less resource-heavy window manager-- so they must be doing something right.
I would be really interested to see if it's possible to get compiz-fusion running in conjunction with E16 to take advantage of the nice compositing effects of compiz. I don't see why I wouldn't be able to, but I haven't gotten around to trying yet. I will probably post again with how that turns out.