I do storage at work and while none of the NAS systems are comparable at all to the kinds of things I do at work I've occasionally dipped down and poked around at the NAS offerings. The main problem with them is they are software not hardware raid, which is why in tests RAID 0 isn't 2x in speed over RAID1 and RAID 10, which is the best one to use if money is plentiful, is typically really horrible. It makes sense, though, a decent RAID card is the same price as these whole devices!
In any case all that to say I was impressed a year or two ago when I looked into all this in detail was Synology. I don't think they are known quite as well, but especially their higher priced models actually perform reasonably well and have a decent processor.
Because I work with high-end storage professionally I've become a storage snob and the only prosumer nas I would get is one of the Synology top of the line models or alternatively I'd just build one myself.