I'm running two and it is so very worth it. Takes a little getting used to; but once you've been with it for a little while, it is difficult to go back. Closest analogy I can draw offhand is to tabbed browsing. When I was using IE I didn't mind not having tabbed browsing all that much, once I switched to firefox, I got hooked on tabs and now I couldn't surf without them. A second monitor is also wonderful for little windows that need a fair bit of watching (Gaim, Motherboard monitor, Bittorrent, etc.)
I should note, though, that I didn't actually pay anything for dual monitors, just hauled up another scungey 17incher from the basement and plugged it in. I might be slightly more demanding if I were thinking of shelling out for a second 2100fp or soemthing of that sort.