This page holds some honest reasons from user of FVWM why they use it, and what they like about it.

I like FVWM for its speed, customisability, and the fact that it looks a lot like MWM without having to do any work. :) – ThomasAdam

I use fvwm because it’s stable (even the ‘unstable’ version), and light but, above all, because it gives me the basis and mechanisms to produce an end-result that most closely matches my ideal window manager. (And if that ideal changes, fvwm can be changed with it.) – slakmagik

If you start with a simple setup and fiddle with it now and then you’ll end up with everything you could want in a desktop environment. It’s endlessly configurable, and at this point I can’t imagine using anything else. – jimkalb

I have been using Fvwm for years. I ran across it when I first started using linux and wanted to get away from the larger desktop environments. It is light weight, reliable and customizable. In the 10+ years of using Fvwm, I have had few issues with it and am able to adapt my configuration as my needs and tastes change. – jaimos