I've got a OCZ 700W with the Active PFC protected by an APC 1500KVa that isn't, to my knowledge, anything close to having pure sine wave. I was reading about this, too, and thought I'd spent $150 on a UPS that was going to fry my system somehow. Turns out it was nothing but a bunch of people freaking out about what seems like nothing. I've had my build for 4 months (see my sig) and the UPS will hold it during a power outage for about 30 minutes which is incredible considering the hardware I have. I've had 4 outages (3 line fuse trips and a pole that broke and took out the whole neighborhood), two of which fried components on the only PC that wasn't protected by any form of UPS - my HTPC. My APC has held steady and hasn't had any adverse effects on my PC yet.
My opinion is buy one strong enough for what you need based on the hardware you have. I work from home, so 30 minutes is huge for most surges and minor outages and I've had no problems running on my 8,000Kw generator (which is definitely NOT pure, clean power - I had to adjust a setting on the UPS to prevnet it from freaking out on generator power) and I've not had a single issue with my computer. Honestly, I really think all this PFC stuff, in relation to a UPS, is overblown. Not only that, but have you looked at the cost of a "pure" UPS? I guarantee you're not getting a 1500KVA UPS for $150 if it's of the "pure" variety.
Of course, that's just my 2¢ - YYMV.