Dell announced the Axim X51 series today and my full review of the X51v unit and Windows Mobile 5.0 should be going live on Geek.com soon. I received an engineering sample and there were a few buggy issues with the first ROM I had. I then reflashed it (took two hands and the stylus in my mouth to do so) and it seemed to fix the issues. I did not focus on them in my review since the units shipping to customers have the newer ROM.
The two major improvements over the X50v were the 256MB ROM and the installation of Windows Mobile 5.0. X50 owners will get the chance to load WM 5.0 on their devices so if you own one of these there really is no compelling reason to pay for the upgraded model. Another major improvement I saw was in the battery life. I saw up to 50% improvement and in other reviews I saw reports of 15% and 25%. There may have been issues with the Spb Benchmarking utility I used, but there is definitely a battery life improvement with the same capacity battery.