I've got the PM 622 Ca Reactor on my 220gal system and I'll never change. However you'll find that most people (2:1) on this site prefer 2-part dosing. Do your research and decide on what will work best for you. Also, have a look in the buy/sell section. I think there are three calcium reactors for sale used right now. If you're looking for a deal that would be the best way to go.
So it's 2010 now. Who still uses a calcium reactor and who's all on 2-part dosing?
http://www.canreef.com/vbulletin/showthread.php?t=66438