Hi,
i upgraded several systems (from 4.1.X, 5.1beta2) to 5.1.2 with no problems - NO problems at all. It was the easiest server-upgrade, i have ever seen; 10 Minute job.
The new 5.1.2 seems to be improved (besides the things documented and stated in the Help-File) in the DNS-resolution. I am now able to reach my external website at the provider again

; this was merely impossible with further versions.
I wanted merely the "allinone" Samba 2.2.2 support, which seems to work well also. These several patches i made to the 5.0 version were obscure to me.
You also gain the latest kernel and improved hardware support. No reason not to upgrade.
Alex