I did have a scare on Wednesday. I could not access the Internet using Pocket Internet Explorer (PIE), however I was able to get mail. I am not sure whether the problem was with T-Mobile or Windows Mobile 6.1. I was a little concerned about the United Airlines PocketFly timetable application I had just installed that morning, because it updates PIE. I ended up rebuilding without the United software and on Thursday PIE was working again.
All seems to be working well now including the GPS. I have not seen any major performance improvements yet . However at this time I do not see the need to use a custom ROM build. The HTC build has all the basics and with Kaisertweak, adjusting the environment works very well.