I think two monitors are sweet.
I am going to get a second, smaller, LCD to go with my current Dell UltraSharp 2005FPW 20.1" Widescreen LCD when I have the money. I can't decide if I want a 17" or a 19" monitor, though, because I don't know how tall my current LCD is, and I want them to match up height-wise. It really does depend on what you do and how much money you have to spend. It also depends on your graphics card to some extent. I multitask like crazy, so I can see obvious benefits. However, there are a lot of people out there who, like many dual core users, get two when they didn't really have a need for them.