Downsides/upsides compared to what? The general consensus is that Windows ME was a mistake
(ME = mistake edition)...PC World magazine listed it as one of the worst Windows release ever, and the 4th worst tech product of all time.
It is unstable, unreliable, and Microsoft no longer supports it. If at all possible I would avoid it.
Why would you go with ME instead of a more recent release like XP or hold out until Windows 7...or heck go with win2000. Just avoid ME unless you like fixing it constantly.