well, I will just tell you my experience.
when i was using win98se, i have to reboot the system at least once a day or two. otherwise the memory leak is terrible. also, with normal use there will be a crash or two every week.
i have been using winme for 2 months, and i haven't reboot it once, and it hasn't crashed once on me neither. i think it is quite stable, though it is no match with win2k.
speedwise, it is the same for me. i don't see anything run faster or slower.