I ran 4.0 via Cyanogenmod 9 on my Nexus S 4G for weeks, and unfortunately the bugs and constant app crashes got so bad that, as much as it pained me to do so, I went back to 2.3 yesterday. I love Android 4.0, but there doesn't seem to be a good, stable implementation of it yet.

Example issues:

  * Random reboots
  * Choppy playback in the Music app 
    (Amazon MP3 worked fine)
  * Random app crashes, particularly 
    games (possibly due to lack of official 
    4.0 support)
  * Poor GPS performance, occasionally taking 
    15-20 minutes to acquire a position (typically 
    solved more quickly by rebooting, but it takes
    several minutes to know there's a problem in the 
    first place)

It's difficult to blame the app crashes on the OS if the app doesn't claim to officially support 4.0, but that's still reason enough to wait.

I'm eagerly awaiting the day that CM9 is released, or MIUI releases their official 4.0 version, but until then I think it's better to stick with what works.

Edit: Formatting.

I've had ICS on my Nexus S for a few months now and I've not seen issues like that. I'm just using the standard Google image (AFAIK, my brother did the upgrade for me - Android 4.0.3, kernel 3.0.8-gb55e9ac). I do see a lot of battery drain when I'm using the GPS, and I don't play games, but no crashes.

I've been using this ROM on my NS4G for about a month:

http://forum.xda-developers.com/showthread.php?t=1364221

In my experience, the stability and performance of various peripherals like GPS and the WiMax radio are about as good as the stock 2.3 ROM. I haven't even noticed a change in battery drain...

I'm running the same ROM on a NS4G also for about a month. It feels like a new phone and has been awesome in every way.

Have any good ROM suggestions? I'm looking for one somebody with first hand use of it can sell me on.

Definitely CynanogenMod. Probably (I haven't checked them all) the most complete and best testet Rom out there. Available for nearly every major Android hardware.

If you have a Droid X and can live without the camera, EncounterICS is unbelievably good.

It is very phone specific for ICS right now. When Cyanogen 9 comes out there will be more 'standardization.' For now, I'd look up the dev forum for your phone on xda-developers and see if there are any good options.

There is only an alpha 1 version of 4.0 (CM9, in fact) out for my phone, and it still has severe bugs.