For most, this really should be a no brainer, and they should get it, if they are due for an upgrade. It is easily the best deal of the year(and next year until the next nexus device is released, if it is similarly priced).
I however choose to believe that Google will continue this aggressive pricing at least next year, at which point I'll be ready to exchange my GN without a doubt.
If you have followed the guide and flashed the update.zip in the end, it should be fine. Easiest way to find out is to see if you have the superuser app when you look through your apps. When you open titanium backup you should get a popup from the superuser app asking if titanium backup should be allowed su.
You should not go with the ota update when you are rooted as your wifi will stop working(reported by others not own experience) If you want a decent (android 2.1) 1.21.405.2 rom with root and no issues, i would prefer "mcr r3.1" by paul. It works perfect, and you can just dl one of the prebakes in the link below, and you can also have it with a2sd+ which i personally prefer :) good luck.
Well as you have already installed 1 update, your rom version would probably be 1.21.405.2, and hboot will be 0.80. You can see this in settings and info.
If that is the case, you can't flash those roms, as you are not allowed to downgrade on non-rooted phones. You have to either find a rom with a version number matching yours or just do a factory reset, which i would suggest as the first thing to try as you are not rooted. It's done in the settings menu under privacy.
edit: Just found a newer ruu for you to flash if you indeed have the above mentioned version installed: http://forum.xda-developers.com/showthread.php?t=684259