I have experience with both the Monaco Optix Pro system and the Gretag Eye-One Photo system . . . and they both perform well. Monaco actually created a better profile and has the option to tweak curves in RGB after the profile has been made.
The Gretag Photo setup is nice because the device is actually a spectrophotometer vs. the colorimeter of the Monaco package. With a spectrophotometer, you can create profiles of other devices, like your ink jet printer. Gretag also sells a monitor only package that is comparable in price to Monaco.
My advice is Monaco based on my experience with calibrating several Apple 20 and 23 LCD displays . . . if you want to just calibrate monitors. If you want to get deeper into Color Management, the Gretag Photo setup might be more useful, however it will cost you about $1,500.