In my experience, this isn't true; the output of my UPS systems is cleaner than what's coming from the wall outlet. Ocassionally this is really irritating to me; they tend to eliminate X-10 signals I use for home automation.
I'm not buying the el-cheapos (the most inexpensive one I own is a Powerware 5115), but I don't have anything over 2000VA. I have 6 UPS systems in my home, all 1400VA to 200VA. None of my home theater gear is connected to any of them, but it's not due to any concerns about noise (I just need the backup power elsewhere).
For the whole enchilada, I like the Powerware 9xxx series (the black ones if it's going into living space). Not cheap, but I'd definitely buy a pair of Powerware 9125 1500VA UPS systems before I spent $1500 on Monster's top-of-the-line line conditioner. :-)
