I have not flashed one yet, as I am support for the wife's phone, and want to get things settled before I flash. Both are rooted and unlocked in preparation though.
The ROMs out there, for the most part, are modified Sense... so you will not lose Sense. They just get rid of the ATT apps, unlock tethering/wireless tethering, add the advanced power menu. Tweaks mostly. Looks like a few have AOSP tailorings if that is your bag, CM9 is still early alpha stage, and that will be a nice option once it gets things working.
For now though, mostly tweaked Sense ROMs, which is fine. I have been looking at stock ICS for long enough, that Sense simply looks more polished. I would seriously hesitate on going to something like CM on this device, as you would lose all the cool camera features. The CM camera is feature poor by comparison. The quality of snaps might be the same, but no more taking snaps while taking a movie, or built in HDR, etc.
This link is where all the hacking is, and you can click through the ROM posts to look at the OP. Usually screenshots and feature lists so you can see what is what. Fair warning though, that reading the feature lists is a gateway to a flashing problem
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One X Android Development - xda-developers