I too just got a Donnelly auto dimming temp/compass rear view mirror.
I had it put into my 2001 X that was originally purchased in April '01. It was all part of the deal for me to buy the vehicle so I'm not sure how much it costed exactly, but it was around $300 (Can$).
It works great and as a bonus it actually helps block the sun from coming in over the top of the mirror (where the sun visor doesn't help) because it is taller than the standard mirror.
The temp and compass seem very acurate.
The autodim I don't know about. I haven't had it too long and nobody's had their brights on behind me yet. Also, the Xterra has tinted back windows that already reduce some glare.
Here in Calgary weather can change drastically during the winter months due to chinooks so it's nice having that temp guage.