Once you go stock Android, you never go back. I love my Nexus 7 and the frequent, direct OTA updates from Google. My AT&T Samsung Galaxy S2 got an update in the last month that turned it into a freezing, battery-draining, glitchy machine with no patch to fix it in site. I will never buy a non-stock Android phone again.
My first Android phone was an HTC Hero. It was vastly improved when I replaced the moribund HTC-customised OS with Cyanogenmod. That's why, when I needed to replace it, I went with a stock Android phone: a Nexus S.
However, whilst my Nexus S still works fine, Google have effectively abandoned it - it won't get any more updates - so one of my projects for this weekend is to root it and try out Cyanogenmod on it.
This is exactly why the "you can buy another great Android device" argument falls face first into the ground for me.
How long can I expect my (at the time) top of the line phone to continue getting the latest OS updates? Additionally, how many (if any) phones get thoroughly tested for OS compatibility and performance for new Android updates? Do I really want to take an unnecessary roll of the dice?
I realize this isn't exactly what you're asking, but most all capable Android devices end up being able to run far-future software via the custom ROM community. My old HTC Sensation (launched May 19, 2011, running Android 2.3) happily runs Android 4.2 thanks to the Android developer community. Even my old underspecced Nexus 1 can run 4.2!
The OTA support may not be there, but that doesn't mean you're hard-stuck with an older version of Android on the device.
