Although I didn't install it, the Kubuntu live-cd appeared to work fine on my 5021 (the 5020 is just a series, not a particular model). I'm running Slamd64 (a 64bit unofficial Slackware port) though so I can't really comment further.
I'd recommend you look up any Acer Aspire 502x's on
TuxMobil and / or
Linux On Laptops as there's a little bit of extra configuration you need to do to get everything working (mostly the wireless), regardless of the distrbution you pick.
In terms of hardware, pretty much everything is supported on the 5020 series except the card reader, but there are various projects working on trying to produce drivers for it (look for tifm21 and tifmxx on berlios.de). Also, I've not had much luck getting suspend to work either (I think this is a common problem with this machine).