Jürgen Weichand, author of the WFS 2.0 support for the Quantum GIS, recently published his master thesis (German only) on the topic of implementing and using INSPIRE DownloadServices.
While a large focus of the thesis lies on the client side (the WFS 2.0 plugin for Quantum GIS was developed in the context of the thesis), some server side solutions were also evaluated. Since UMN mapserver does not have a WFS 2.0 implementation, only GeoServer and deegree were considered OpenSource-wise, the GO Publisher WFS was chosen as closed-source example.
Looking at table 6.7 on page 67 it seems we're doing pretty well. I'm sure the upcoming 3.2 release will improve the situation even further. And it's one of the first publications explicitly mentioning us at Occam Labs as developers of deegree!
We probably could have made an even more prominent place in the thesis if we had a nicer web administration console ready (chapter 6.2.6 mentions this as the probable reason why GeoServer was chosen for an example implementation, simple features only!). At least with the deegree handbook nearing completion it will allow future thesis writers (and all others of course) to better understand how easy it is to set up a WFS serving rich features with deegree.
Edit: I did not mean to imply that GeoServer can't serve complex features (as Andrea pointed out it has been able to do that for years), it's just not as easy to do using the web interface (which Jürgen Weichand also pointed out in this thesis).