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.
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?
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.