Pure Google is overrated, the OEM skins are getting better and you can just use a launcher anyway. ICS still isn't entirely coherent design-wise, and 3rd party skins are not the laggy, ugly system-hogging beasts they used to be based on the reviews I've read. The best reason for picking a Nexus device has always been the implication of 1st priority in OS updates, and Google screwed that up. When you have phones with OEM skins getting ICS before some Nexus S phones, you've basically thrown the biggest reason to own a stock Google phone out the window. With the upcoming awesomeness of the GSIII and HTC OneX and all the other superphones coming out, I'd be hesitant to give that up just for a "pure" Google experience that's just not indisputably better any more.
While the nexus s fiasco sucked, it still got ICS in the end (fully hardware accelerated). I still feel much more confident that any nexus device I get, will be supported for at least two android iterations past the one it ships with. While going HTC, Sammy or Moto means maybe one update, depending on how flagship the device. How many of these phones that are only JUST getting ICS this year, will also get jelly bean? Or if they do, how long will the wait be. Of course, there's always custom roms right? Well, that brings me to the next thing...
The modding community for the pure devices also seems to be the best. Less issues with drivers and such means getting new AOSP built updates with fully working hardware. I can't tell you how many times I've installed the best AOSP roms for non nexus phones that always have that "but this doesn't work" catch, at least until there's an official kernel update that can be the basis for the new rom, or suitable drivers are found for the version of android being used.
Also, no waste of space by using those unchangeable hardware buttons.
Oh, and those 3rd party manufacturers skins are still gaudy and even less coherent than vanilla, even if it's less so than it was in the past. Though they usually bring some admittedly nice features (that can be had with custom roms on the nexus side anyway).
Still, with the improvements made on the hardware AND software end (with ICS providing a fully accelerated interface) when it comes to 3rd party manufacturers, the gap is closing. I have no problem recommending a ONE X or GS3 to someone who's stuck between those phones and a GN.
Also notice some screen retention switching between apps sometimes.
