Probably I'm not supposed to talk about it here but what the heck, Dell abandoned me on this matter so... I installed WM6.1 in my Axim X51v and I'm glad that I did it. It's like I bought a new Pocket PC. My Axim works faster than when it was using the so called "legal" WM5 ROM from Dell.
One of the reason why I have not done it before was because many of these 3rd parties ROM do not work well with .Net applications and in my work with Tweaks2K2 that's really important. But these ROM have gone a very long way from the point of view of quality. The one I'm using does not have to envy anything to any of the ROM released by Dell in the past.