In the basic HTML mode, yeah, Gmail works great. I've used it a lot.
It's not fast on an 030 but well, it wouldn't have been fast on a 386 either.
YAM is still my fave email client ever too

Otherwise yeah, compatibility to standards is a problem, but are those OS issues? Is there an inherent reason we couldn't have, say, a more compatible version of APDF, or simple export to .doc format etc in an amiga app ? (assuming there were people to code the damn thing and people willing to sponsor it)