I can't comment on Nexenta's AD/CIFS implementation as I've just started using that feature, but we have been using it as our primary storage for just over a year now, with an appliance here in the office and one in the datacenter. I assume you're talking about the NexentaStor appliance platform, and not the Nexenta distribution.
1 - It has been great to use, though we are primarily using it for NFS. Depending on the system configuration, it is very fast and reliable. In our (crappy) office, we have had quite a few power outages, and it never skips a step. We use it with both OracleVM (Xen) and VMware esx, it has been great for both. The only caveat, at least with NFS, is to be sure to have dns setup and fully populated with all your servers.
2 - I would recommend going through a vendor, having both built our own (in the office) and purchased from a reseller (PogoLinux appliance in the datacenter). Everything works fine on the one we spec'ed out ourselves, but performance could be better, and we forgot a couple things up front. We didn't spec out OOBM on our home-built one, so that has bit us a couple times and necessitated a couple late night drives to the office. We also chose a storage controller (sata JBOD) that I've since heard is very slow compared to other stuff out there.
Our experience with PogoLinux has been great, the solution we created them has been top-notch performance wise, and their support is a great addition to normal nexenta support as they have a lot of hands on experience with the hardware, software, and storage implementations in general. Price-wise, it was very close to building it ourselves, so the extra 2% or so we spent with PogoLinux has been well worth it.