I have two APC units.
The old one is from my dual core build, a 400W UPS, and the new one is for my 2600K build, a 750W UPS.
I could run everything off the 750W (12 mins runtime), since I didn't go SLI (no game can faze my PC anyway), but I run my monitor/modem/printer/cable box/ off my 400W, which give me more time overall when the power goes out.
I'm in a highly populated area, but we get alot of brownouts, and probably 2-3 blackouts per year. In the past week my UPS even saved me from my own stupidity. I have a line running from behind a standalone freezer, and while cleaning the coils of the freezer, I apparently pulled the plug enough so it was hanging and falling out from the weight of the cord (high outlet).
Later I'm sitting at my PC, and *beep beep* "Low Volts" and you have '24 minutes'. Hmmm... all the rest of the power is on, wtf. Checked the plug (first troubleshoot of any problem), and plugged it back in.
UPS ftw to put it simply. Insurance and even awesome when it kicks in. Only have used APC, so I can't comment on other manufacturers though. Not really happy with APC's current software.