I tried it twice… trying to use the cooked Chinese ROM (translated to English) and found that the device tends to slow down due to the very sad amount of free RAM available. Mind you that WM6.5 is slightly more resource intensive. I found that I could only run two programs at the same time when on WM6.5. It could have been the version I flashed, but seriously, 64MB RAM cannot allow you to do much. It was so bad that at times I had to clear all programs in order to use the camera!
Sad, but true. On the other hand, remember that it would not be a problem if you are not intending to use a resource hogger like I do (I use SPB’s excellent Mobile Shell 3.0) which makes WM more user friendly and useful. If you are fine with the default home page, then by all means, upgrade it to 6.5. Fundamentally, I see that there is no real difference between 6.5 and 5. You can still use most programs (since they are backwards compatible).
There is the lost of some default input types (on the 21725 version I think), which I have no problem with, since I use Resco’s Keyboard. In the end, the only reason why someone would use Windows Mobile in this generation of iPhone OS & Android OS is because of its customization and tweeking capabilities. The variety of software packages that you can try out is really limitless 🙂
So if you want a smooth performance, retain WM5 or maybe just upgrade to WM6.1. If not, be prepared to lose out on using 3rd party Home default programs 🙂 The choice is yours.