Reliable, solid, wide range of options (number of drives), RAID6 on the models that fit enough drives, full disk encryption, lots of plugins to expand the capability, RAM upgradeable in some models, gigabit ethernet.
They are not the cheapest, but I always figure good equipment pays for itself over time.
QNAP
http://www.qnap.com/uk/index.php?lang=en-uk&sn=5050&c=2485&sc=2491
Reliable, solid, wide range of options (number of drives), RAID6 on the models that fit enough drives, full disk encryption, lots of plugins to expand the capability, RAM upgradeable in some models, gigabit ethernet.
They are not the cheapest, but I always figure good equipment pays for itself over time.