For what you get, Millenium Edition is ridiculously slow. There are a few more options than in 98, and far more annoying things that aren't in 98, plus it is much much slower (excluding the quicker boot time).
Windows 98 is blisteringly fast on a 350 with 96MB RAM. Millenium Edition makes this system look completely obsolete, far more so than Windows 2000 Professional in fact.
I installed 98 on top of ME once. :Q
I had a ME installation and I just went to DOS (boot disk) and installed 98 on top of it... Needless to say, I didn't keep that setup for too long. It was horrible.
98: Extremely fast, good for the average Joe.
ME: Slow, worthless for all, NOT "the gaming OS", FAR from it.
2K: Excellent stability, slightly slower than 98, but very much worth the move depending on the user.
-RSI