Is Lion server really a valid server for small business? I'm considering to replace my existing windows based server on my small business (&lt; 10 employees) with a mac mini server with Lion Server installed. But after a look at the App Store, I noticed a lot of 1-star reviews labeling this OS as "a toy", "unstable", "downgrade with respect to Leopard Server" and so on. My usage for this would be essentially for e-mail management (but using Kerio to have exchange support), calendar, internal web site (web apps), account management, some centralized apps (to be accessed using VNC), iOS based devices management, Windows virtualization to support legacy apps (essentially for accounting), VPN.

I'm not asking for a complete review of this OS, this is not a right place, bust just a short comment from the users in the community that had the change to work with this new OS. 

Besides, and this is a bonus question, do you have some experience in using Lion server to manage centralized Xcode development? (we currently use our own MacBooks and we keep in sync using Git, I'd like to know for different approaches based on OSX Server).
I've never tried any of the Mac Server products. But AnandTech has a pretty good in-depth review of Mac OS X Lion Server. A few of the conclusions:


  ...server administrators will be happy to know that, in spite of its deeply cut price and consumer-friendly distribution method, Lion Sever stacks up favorably against Snow Leopard Server.


and 


  OS X Server is most useful in a handful of different scenarios: the first is that you have a small network that’s in need of a full-featured but easy-to-manage and simple-to-license server product. The second is that you’re managing a network of any size that used to be all-Windows, but hosts a growing number of Macs (this is often the case in education, for example) - OS X Server knows that it’s going to be finding its way into a lot of Windows houses, and as such it integrates fairly well with existing Active Directory setups. The last is that you have a bunch of iOS devices flooding your network and you have no idea what to do with them - iOS management may be Lion Server’s ace in the hole.


(emphasis mine)
Seems to fit your intended usage pretty well.

About your other question:


  Besides, and this is a bonus question, do you have some experience in using Lion server to manage centralized Xcode development? (we currently use our own MacBooks and we keep in sync using Git, I'd like to know for different approaches based on OSX Server).


Keep using git, use the OSX server as central repository (maybe consider buying GitHub Enterprise or using an free/open source alternative for managing your git interaction). Install a CI-tool (Jenkins or similar) on the server and use it to create automated builds. Should work quite nicely.