Personally I just use the box of like 3 chemicals from the local pool store and have always opened to crystal clear water.
I was recently checking this out and came across this from a really well known genius posted on another site about the same product:
"chem geek10-07-2007, 02:04 PM
You would be better off spending the money on PolyQuat 60 algaecide and chlorine. The WinterPill appears to just be enzymes, and these will help speed up the breakdown of organics to prevent scum lines in the pool, but it won't kill bacteria or algae by itself. It still needs a strong disinfectant or oxidizer such as chlorine to do that or at least an algaecide to inhibit algae growth.
The easiest thing to do in the winter to keep the pool great upon spring opening is to add chlorine periodically during the winter, though far less frequently due to the colder water temperature. For closing, you can shock the pool with chlorine and then the next day add a strong "algae killer" dose of PolyQuat 60 algaecide and then close the pool. There are somewhat mixed results from doing this without any maintenance during the winter, but for most it works OK. Adding some chlorine just when there's a thaw in the spring will also help prevent an algae bloom."