The main problem with most UPSs for computers is the sine vs square wave problem. Computers have no problem taking square wave approximations of the sine wave because they run on DC current internally anyway. So the power goes through a rectifier before it leaves the power supply and goes into the vital components of your machine.
*However*, the electric motors that run your filters are highly susceptible to the square wave approximation. They may run slower than normal or have other weird effects, but mainly the problem is heat buildup. The motor will overheat when exposed to the square wave for extended periods and could lead to damage of your precious Eheim or whatever.
It shouldn't have any effect on lighting (although the ballast might choke on it, I don't know... any double Es out there?) And since your heater is just a fancy resistor, it will do just fine. Although being a resistor makes it a terrific energy drain. Picture those CF displays at the home store where they have the spiral CF vs an incandescent light on a power meter, you get the picture...
If you have a lot of tanks and are prone to outages, a generator is the way to go. Even a little putt-putt type that you run outside with extension cords would be worth the investment given how much time and money you've spent on fish and equipment.
