I calibrate now - I just got my Sypder 2 Express this week. It made a big difference to my MacBook Pro screen, and less difference to my external monitor (an old-ish Samsung LCD), but was still definitely worth doing.
My prints had never been way off what I was seeing, but there was always that nagging feeling when I was making adjustments in Aperture that I was actually making things worse not better.
That was especially true with skin tone/ white balance combinations where minor changes can make big differences. At least now I can be more confident that what I'm seeing is what's actually there.
And even thought the Spyder 2 Express doesn't technically support creating multiple profiles for multiple monitors - in my case, my laptop screen and my external LCD -
I found a simple workaround for that.