1. That really depends on the ROM. Most ROMs (especially those available on these forums) have "issues" threads that spell out all the issues that people are having. Be sure to research them before flashing to them. However, generally, the ROMs are as stable as the original ROM... sometimes even more so. Now, if you overclock... that may or may not be a different story. YMMV.
2. Yes, but it can be tricky. You won't be able to do an OTA update to the stock 2.1 ROM, and most update.zip versions you'll find are rooted, so they're not QUITE stock, and you'll need to unroot them. It will take a few steps, but if you do some research and follow the directions, you can flash back to the stock ROM, and remove the root if you need to.
3. It seems that the OTA 2.1 update is nothing more than the leaked ROM that's been out for awhile with a few changes to lock down the ability to root it. Fromt those who have tried both, most report that the ROMs available are far superior to the OTA update that's being pushed.