I have been pretty pleased overall with my 2005 Xterra. I have over 140,000 miles on it and I'm just now experiencing major issues with it. I've had recurring issues with the O2 sensors going out more than once and now I need a new catalytic converter. I wasn't aware of the transmission and radiator issues on the 2005 models until I read this post. As it turns out, I too have an issue with a cracked radiator leaking into my tranny and now I need to replace both. I thought it could have been due to a minor fender bender a few months back but now I'm not so sure, especially because I was hit in the rear.
Seems like I got lucky to have made it as far as I did on mine.