[Asphyx]

Well any WM SYS will constantly be changing and improving (AND Breaking) code.

MS doesn't stop updating an operating system just because it has been released on a device.

That said I have read that the 23017 sys is the most stable version of 6.5 to date.

As for where they come from...some are ripped from actual phones, Others are leaked by MS developers (I presume this don't know exactly who!).

XDA is a good place to look for sys resources and origination info.

I suggest if you really want to play around with different sys' you download a good visual kitchen which will let you cook a rom for any sys that gets released in the future.

Not every sys is as buggy as they would appear. In some cases it is not the sys that is really at issue but the software cooked in that is incompatible with some change made to the way the OS works and returns info.