I will have to respectfully disagree with takeshi.
Its a electronic piece of equipment, so the possibility of error always exists. Take a look at the forums to see how often official releases of software screw up phones.
I will agree that when it comes to putting custom roms on phones, the community has come along way in making the process as painless as possible. There are usually a number of ways to recover from mistakes, if you take your time and do some research.
That being said, I did not respond to your question in order to scare you off custom roms, but I just didn't want you to be under the impression that nothing could go wrong.
Happy tinkering.