Well im NOT microsofts biggest fan, so don't think this is easy to say, but, windows me does improve on a thing or two over 98se.
First and most noticeable is boot up...if youre one of those people that loves a fast boot up (to show off or to cuz your impatient) then the $50 might just be worth it heh....
Second, a lot of small things have been tweaked and upgraded...small example...on my previous machine (win98se) when i did a clean install and needed to hook up my @home cable internet, i had to manually set up a lot of configurations...which was nothing drastic since i had written them down, but took a good 25 mins to get everything working fine. In WinMe its a matter of choosing the "Im connected to the internet on a LAN" option and the rest is autodetect. A lot of small changes like that makes it a smarter os...which is what i expect when you are giving up security, stability and overall reliability, you want more than just a pretty GUI.
The worst feature of win ME is prolly the Restore thing. The problem with this overambitious add-on is that it just does not work...period. Everytime I have had a serious problem, restore would tell me casually "Unable to restore to that point"...which is not fair cuz i made constant restore points.
Anyways, there is my two bits.