FVWM is one of the oldest window managers out there, and yet is also one of the newest. It has elements in common with all other desktops, and yet adds new quirks of it's own. It's almost impossible to write anything about it of lasting value, because it's such a moving target.
The old FVWM was very basic. It resembled a souped-up Motif window manager and though I played with it with some amusement for awhile, I dropped it as an uninteresting relic. Next I grabbed Puppy Linux when it came out, and actually reviewed it with the mistaken impression that it had the IceWM window manager. Readers wrote in to correct me: no, it's FVWM-95 (this was on an old blog now long defunct). Huh! On writing this review, I stopped and looked at my own install of FVWM and decided that I should grab the new version, because every time I saw a screenshot of it online, it looked much more advanced than my own creaky version. Also, I saw screenshots billed as "FVWM", but one looked like Enlightenment, one looked like TWM, one looked a little like Gnome... So I grabbed it and the themes and extended themes package, with an eye towards finding out what the heck was going on.
This time around exploring it, I wandered about in it in a dreamlike haze. It can look like Afterstep! It can look like Blackbox! It can look like Windows! Hey, now it's a MacIntosh! Just change the theme, and the whole desktop morphs into one system after another. In fact, when one finally gets to the root of the matter, FVWM can imitate just any old window manager under the sun. Not only this, but due to the advanced theme functionality, you can select to use bits 'n' pieces of many themes to create your own...thing. It turns out that FVWM comes with many desktop programs and controls - taskbars and pagers and slits and wharfs - if it's an element in any other desktop system, it's emulatable under FVWM.
One is tempted to think that the FVWM developers did it this way just to confound window manager guide-writers. OK, you got me! Ha, ha. FVWM has Perl as it's native language, so has always enjoyed an enthusiastic development community. It seems to have evolved a new feature whenever anybody asked for one. It consists more of a goulash of other desktops than it's own method.
Unlike Enlightenment, FVWM's happy chaos is somewhat contained - there is consistency in the root menus and window controls, and the absolute zoo of desktop gizmos are the only thing to change. Follow the icons and trust your instincts and you'll usually figure things out. But expect surprises around every corner.
It's performance is slightly faster than Xfce. I did detect the occasional bug with a theme, a minor one like icon display. Outside of that, it's pretty stable.
I'm absolutely stumped what else to say about it, except that FVWM has been declared dead multiple times only to be reborn again. It has a rock-solid fan base and a giddy development community. It is documented somewhat, each theme is actually it's own window manager/desktop environment anyway, and if you try to explain it too much, you'll end up sounding as incoherent as I do here.
FVWM is recommended for the schizophrenic. Who do you want to be today?