Xorg is significantly lighter on resource use, while using E!.
Matter of fact Metacity used 25MB and E! only uses 10MB.
Infact, that is with a ton of special fun stuffs enabled.
Xorg went from 200MB of resources IN USE with a tons reserved using Metcity, down to 100MB with E!.
Mind you, I was running Evolution, about 50 terms, VMware(with 768MB allocated), Firefox, GAIM and a crap-shoot of other proggys.
Even @ home it rocks.
Oh, I changed Window Managers with out restarting X or GNOME or even logging out. Everything survived.
The only problem I haven't figured out is registering the config stuff with GNOME, so it works from the "windows" thinger in the gnome-tool-center
Oh, also all key-bindings seem to be controlled by E! now... a bit annoying at first, but very very manageable.