But it's lightweight.
If you're recovering a P200 laptop with 32MB of RAM to be used for very light comm duties, FVWM isn't a bad choice.
That said, it wouldn't be my choice. I've got a terminal server that can have displays exported across dial-up + VPN. I needed a lightweight front end. I looked at both FVWM and TWM and felt that both of them would be too different for Windows users to figure out quickly.
I settled on XFCE.
Lightweight, fast, low overhead, and quite usable.
I have been running Fluxbox for serveal weeks now and love its lightening-quick startup time. I'd gotten so used to KDE taking forever to startup after login, that I keep forgetting that fluxbox typically takes less then 5 secs to start up.
I installed xfce last night (via yum), and played around with it. I like it's combination of quickness and features, it's quick, and it's seems to be quite useful. It's got a very good chance of becoming my deafult WM.