Did youll know about this tool here [Only registered and activated users can see links. ]
Its one of the best and important tool used to validate your XML and RSS feeds. Until and unless your feed gets validated with it your feed would not work.
And best thing is that it works on the basis of the W3C standards so untill you pass in this there are very rare chances of your feed working properly.
Just came across this couple of days while coding XML feed and did break my head a lot after it cos it only validates feeds following the new W3C guidelines.