i regularly use music, web browsing, IM, sms, phone calls, calculator, xterm, some games, etc. i think i push the phone a little bit. and i don't have such issues. however, i do experiment quite a bit with software so i end up rebooting every 4-5 days as some of the apps in testing might not be very stable yet. but the OS stays quick and smooth.

what apps do you have installed and what do you regularly use?
it might help to do a screenshot of the processes using "top" when it starts slowing down. post it here.