Calibrating of Mac screens

Has anyone used a screen calibration device to good effect on either an iMac 24 (glossy) or a normal macbook?
will be used for photo editing

Yep, I did it manually on a MBP to get rid of the yellow tint. The difference is night and day...you don't notice the yellow tint until you correct it.

I have used a Syder on various macs, it does a good job

I personally prefer the X-Rite Eye-One Display 2. Before buying this, I tried the Spyder 3; I had an installation issue, and I found their tech support wasn't all that great. So, I returned that and bought the X-Rite calibrator.

Also, the cord for the X-Rite sensor seems heavier; since the sensor hangs by the cord, a heavier cord will hopoefully last longer.

how about the bit of glowing i the corners in black screen? any one know about this and if cal. will help.