[AndresNavarro]

That's what I came here to say. I can't believe the use of the compose key isn't more common. I just remap caps lock (which I don't like) to compose and kill two birds with one stone...

[hahainternet]

On a lot of Linuxes you can use alt-gr for third level keys too, which is handy for «» for example (altgr + z, x) or “” (v, b)

[geofft]

I imagine it's uncommon because it's thoroughly undiscoverable, and not super useful by default for lack of a good .XCompose file. I know how this stuff works and I haven't bothered because I don't want to figure it out again on each machine / desktop environment / etc.