The auto-dim on my 2005 package 6 works superb. (package 6 self-leveling Xenon headlights are also superb, BTW)
Seems that the earlier Prii have better mirrors than Gen3 or 4. Lots of threads and posts about the auto-dimming (or not) experiences when the 3 was new. Mine just plain sux. I don't expect any different in the Prime. If it does dimming in actual practice, it will be a bonus.
You mean Tideland2005 Yes and no. It's the same mirror (made by Gentex). It's not actually the mirror's fault but the fault of the Prius' rear spoiler. In the Gen 2, you could flip the mirror upside down and it would work perfectly because the sensor could sense the lights properly. If it's right-side up, the rear spoiler scatters or blocks the light so not enough of it reaches the sensor. The same Gentex mirror is used in many vehicles. MB and BMW are the only two off the top of my head that manufactures (or at least source it elsewhere) their auto-dimming mirror.
I am missing my dimmable side mirror a lot. I am so much get appreciated it for a night driving. My toyota solara 2006 had 2 electrochromic mirrors, but prius has only one and I have had to pay a premium to get at least one such a thing coming with advanced package only.
Just to clarify, the 2006 Solara in Canada had a driver-side electrochromic mirror as well as the rearview electrochromic mirror. (rather than the typical driver & passenger electrochromic mirrors found on Lexus and other luxury brand vehicles).