I had a Spyder2 for the last 7 or 8 months. It served me well on both my notebook and my dual display desktop, until I recently got a wide gamut 24" LCD. The calibration was 'off' on the wide gamut screen, so I was forced to upgrade.
After waffling on the decision (spyder 3 elite vs eye one display 2) for about a month I decided to go with the Eye One. I just received it yesterday, and did initial calibrations on my notebook (XP MCE) and my desktop (Vista 64) last night. Both went very smoothly, but I haven't had much time to adjust/compare.
The differences on my notebook were minimal, but subtly noticeable.
On my desktop, the differences on my 19" TN LCD were again minimal, but subtly noticeable. On my 24" HP LP2475w (IPS, wide gamut) the differences were very pronounced. I've had it for about 3 months now, with the old Spyder calibration. It will probably take me a week or so to get used to the new calibration.
Just for example, calibrating the luminance to 110, my previous brightness and contrast settings were 46 and 44 respectively. With the Eye One, those settings are 16 and 100 now.