i have the lx...the only thing i really dont like is the inability to use custom ringtones, but i knew that before i bought it. no video recording/playback is an issue too but an OTA update is supposed to be coming in the next couple months to fix that.
to answer your other question though, yeah you can add mp3s to it.
but yeah it's my favorite phone i've ever had...
edit: and cosign with whoever said the slide sucks. i especially hate the keyboard on that shit. and the mood lights on the lx >>>>> all haha