Opinions on RO/DI systems

[Stones]

Just wondering how much of a difference it makes for a RO/DI setup to have 2 DI filters as opposed to just having one.

Also, has anyone ever used a RO/DI system with a "catalytic carbon" filter in it? I've only seen one system with such a filter and was wondering if it is worth the extra dollar they are asking for it. Apparently its used to filter out chloramines...

If anyone could recommend any good RO/DI filter systems as well, I'd like to hear your opinions on how well they perform. Thanks.