The converter will work on plugin files (.esp), but also on the master files (.esm) shipped with the game, and on saved games (.ess).
The conversion to "XML" was designed to be invertible without a lot of prior knowledge in the XML-to-ESP translator, which is why it looks a bit odd! If anybody out there actually knows XML and can make suggestions as to how it would be better done, please do get in touch.
Also, because my intial purpose was to store .esp files in CVS, I've not gone all the way down the road of actually interpreting the data. If you are interested in more of the details of the objects (for browsing or editing) and this version doesn't expand them in enough detail, drop me a line and I'll do what I can to expand it for you. I do plan to cover it all eventually!
To save you the trouble of downloading it if it's not going to do what you want, here is an example conversion of a totally-unfinished mod of mine (about 140K, uncompressed) so you can see what the format is like.