I use a polyquat every other week (the recommendation on the bottle says every week, but I ignored that). I've never had an algae problem in 3 years, but then I only got seriously involved in the PoolForum recently. It looks like maintaining a sufficient chlorine level is enough to prevent algae, but using an algaecide will prevent any "accidental" drop in chlorine, even to zero, from producing an algae bloom. That's the main reason I use it -- I'm pretty diligent about adding chlorine, but I have had at least two times where I let the chlorine drop too far (close to zero) -- and fortunately, had no problems.

Some people are investigating using Borax not only to raise pH, but to maintain a level of Boric Acid in your pool that presumably is an algaecide as well (it also acts a little like a pH buffer, similar to the carbonates). Evan (waterbear) is currently investigating what exactly the "ppm Borate" means on test kits since we currently have conflicting information from different sources.

Richard