They're the best protection available, but that protection comes at a cost: Maintenance.
The most common mistake I see people make is just to install it and forget about it. It has to be removed periodically and cleaned, as well as the paint underneath it. Sand and road grit eventually finds its way under the bra and if left unchecked, can act as sandpaper on the paint. And leaving it on wet...that's paint homocide. You can get away with it a few times, but long term, it's just a bad idea.
I used bras on several of my cars over the years, including my 2004 3i and have always had good luck, but I kept them cleaned and well maintained. If you're not so inclined, don't get one.
As far as damaging the paint..it only happens if you don't maintain it. Trust me..after 5 different cars ranging from 1991 to 2004, I can testify to how well they can work when used properly. If you're a no-maintenance type of person, invest in a clear bra. Pricey, but very convenient. Colgan makes my favorite bra. It fit my 2002 Altima like a glove and kept its shape for years. They're thickly padded and generally high quality.