Quite a few people have one of these and there're so many deal threads with loads of posts on it's hard to keep up.
Here's a place for any queries, problems and suggestions.
I have 2 - One as an HTPC in my front room running xbmc and one in my office running WHS 2011.
This wiki is THE resource for HP MicroServer users. It has an easy to search list of compatible hardware etc.http://n40l.wikia.com/wiki/HP_MicroServer_N40L_Wiki
I have added this for new users who find searching through the 2000+ posts on this thread difficult.
Enjoy your new HP MicroServer and please post your results here and to the wiki site for the benefit of us all.
Here is a extremely detailed and noob friendly guide created by me to do the complete package, Installing ESXi > Installing XPEnology on ESXi :)http://cyanlabs.co.uk/news/how-to-install-esxi-on-n54l-and-xpenology-on-esxi