This is the first I've hear of the sine wave issue for active PFC power supplies. What I can tell you is this. I have 8 computers in my house that I built. All have active PFC power supplies, and all are plugged into UPS's. I highly doubt any of my UPSs are pure sine wave for what I paid for them. I have several APC 1500 Back-UPS RS. They are old. I've replaced the batteries, so they still work just fine, and I've never had an issue with them not working during a power outage or brown out. I'm sure the pure sine wave UPSs are better, but for the average (and even the not so average overclocked multi-GPU) PC, I'd have to say a standard high quality (there are lots of crappy ones out there) UPS is fine.
Someone stated earlier that you don't need a higher power UPS. While that's true, you only need it to last long enough for the software to power down your computer, my experience is that the batteries lose maximum capacity over time, so if you only have barely enough to get your PC shut down, in a year or so it may not have enough power to get the system fully shut down. Other factors can make that worse, like installing software that takes longer to shut down, or just having many programs open to shut down when there is a power failure. It gets worse if you install another hard drive or especially another or an upgraded video card. If for instance you are in a graphics intensive game when the power goes out you will be drawing much more power than you would running word. Not to mention the fact that some games only save at specific points that you have to get to. It's really nice to have a high power UPS that lets you run at full load for 15-20 minutes before you have to shut down. Frequently the power is back on before you have to shut down!
My wife an I have played many games together after the power goes out. We don't even consider starting to shut down for at least 15 minutes. We can usually get to the next save point in Borderlands for example before we have to decide whether to keep playing until the UPS starts fast beeping, which still gives us plenty of time to get shut down, or to go ahead and shut down immediately and go do something else that's fun when the power goes out...
The glow from your monitor can really come in handy when looking for a flashlight too!
Korxax