This weekend I applied the first update from FreeNAS. It is very easy to do. First you need to stop all services in the webinterface. Then, you download the apropiate patch and checksum from their website. After. In the advanced settings of the webinterface you upload the patch, copy and paste the checksum and apply the patch for update. You wait 10 min or less if you have a fast server. The secer will restart twice. After that, you re-enable you services and all is done. Very easy, it is the only thing I had to do, since I configured this server 3 months ago. It is working non- stop and flawless for me.
Nexenta is based on Solaris, which is the home-base of ZFS. FreeNas and ZFSGuru are based on BSD.
It is amazing that FreeNas 8.3 can keep up with Nexenta now. ZFSGuru can´t keep up with the other two and according to the benchmarks the difference is quite big. The makers of ZFSGuru are looking now if there is a benchmark error or it is because of the version (0.2 Beta 7) used in the benchmarking. Since ZFSGuru comes with a full version of FreeBSD, there are a lot of possibilities of tweaking with the commandline.
However, I personally prefer FreeNas on a production system since ZFSGuru is still more in development (but stable according to many users), missing alot of features in the GUI, and they (still) are missing a good manual (like FreeNas 8) which makes life for noobs like me easier.