1) If you have SuperCID, installing the OTA may result in a brick. Its not completely clear yet exactly what the brick conditions are. People with SuperCID and S-off appear to be okay. Which makes sense, because the issue is with CID check, and by definition all security checks should be bypassed with S-off. Bottom line: It is not recommended to install the OTA if you have SuperCID, unless you first do S-off.
UPDATE (March 21, 2013): The 3.18 RUU was just posted on HTC's website. It appears to also brick the phone if you are SuperCID and S-on.
2) Nobody has yet posted that the X-factor root exploit works or not on the Jellybean OTA. HTC patched the previous root exploits with every other OTA to date (1.85 and 2.20), so its very possible that they have done the same with X-factor.
If this is the case (X-factor does not work), and you are not already bootloader unlocked and/or S-off, and you install the OTA, you will be stuck without root until a new exploit is discovered and posted. This happened with the 2.20 OTA, and many folks here were stuck without root for (I believe) 3+ months until X-factor came out. If you are not already rooted (bootloader unlocked, S-off, etc.) and think you may want to be, it is highly recommended you do so before installing the 3.18 OTA.
UPDATE (March 10, 2013): Its been confirmed that X-factor does not work to root on the 3.18 OTA. Root (unlock bootloader, S-off, etc.) before the OTA as warned above, or you're screwed.
UPDATE (May 17, 2013): A root method has been achieved for 3.18. See here:
3) OTA Rootkeeper seems to be working for some people, but its not clear. It didn't work on 2.20, so don't assume it does. If for some reason you are rooted but not bootloader unlocked or S-off (for instance, you are on 1.73 or 1.85 and used the corresponding root exploits), its recommended you unlock the bootloader and/or S-off before installing the OTA, to ensure you retain the ability to root.
4) Frankly, I don't personally recommend installing any OTA if you are rooted, on a custom ROM, etc. If you are rooted/bootloader unlocked, and want to try the stock AT&T Jellybean ROM, just flash the rooted stock ROM here (flashable in recovery). Better yet, the first custom ROM based on 3.18 is already posted here, with others sure to come.
Some people like to install the OTAs anyway. To each his own. Just be aware of the warnings as stated, do some more research, and plan accordingly.
5) Root and custom recovery interfere with downloading and installing the OTA. Your best bet to install the OTA is to return to stock to properly install the OTA, by relocking the bootloader (if unlocked) and running the RUU.
6) The situation is very fluid. The OTA is just starting to roll out, and not many people have installed it yet (nobody had posted that the tried X-factor, etc.). So some of this information may not be complete or totally accurate. As events unfold, and users post their experiences on this forum, I will try to update this post accordingly.
Cheers and good luck!