Another way to come at choosing a NAS is to look at it how you'll primarily use it. This is where you have to be really honest about separating the "must haves" from the "would be nices".
General File Storage, Serving, Backup
These needs can be met by any NAS. The deciding factor may be how your prospective purchase handles backing itself up. If you're using a NAS as primary data storage, i.e. the files are not stored on client devices, then NAS backup is crucial.
If primary storage is on client devices and the NAS is the backup, then backing up the NAS is more a matter of recovery time and convenience vs. preventing data loss.
Backup support varies and requires detailed homework to determine whether a product meets your needs. Options include attached, local network, Apple Time Machine and cloud backup services.
Econoboxes usually will back up to an attached USB or sometimes eSATA drive on demand and might also support automatic scheduled backups. Server Killers, Kitchen sinks, Classic RAID5s and Compromises will support both attached and NAS-to-NAS backup, both immediate and scheduled.
In the past, NAS-to-NAS backup usually worked only within a manufacturer's own product lines. This is still the case for Buffalo NASes, but most other manufacturers support backup to and from generic rsync servers in addition to their own customized versions of rsync.
If you want the most flexible NAS-to-NAS and client-to-NAS backup options, get a NETGEAR ReadyNAS. They can do immediate and scheduled backups to and from most anything and have reliable emailed backup reporting. Iomega NASes are the next most flexible, supporting backup to and from SMB shares and rsync servers.
Media Storage and Streaming
These duties can be supported by any NAS if you don't need UPnP / DLNA or iTunes serving. NAS performance of even Econoboxes has risen to the point where the 20 to 40 Mbps (2.5 to 5 MB/s) requirements of 1080p media streams can easily be met for even multiple streams (see Can Your NAS Do Two Things At Once?) . And there are plenty of entry-level NASes that throw in a basic UPnP AV server and usually an iTunes server, too.
If you want media streaming to iPhones / iTouches or Logitech Squeezeboxes, web-based photo viewers / slideshows, or anything else special in the media serving department however, look to Compromises, Classic RAID5s and Kitchen Sinks.
Server Replacement / Special Applications
If you're thinking of retiring a heavily used Windows or open source server, then you need to be looking at the high-end Kitchen Sinks or Server Killers, although there are some good choices in Classic RAID5s, too. But even then, these products have their limits. They might work fine for file serving and backup for small offices of a few dozen employees. But they also might crumple trying to support even a handful of users simultaneously hitting a heavily-used QuickBooks database. We don't test multiple-user performance. So you're on your own here.
So for those of you who just skipped to the end, hoping to find a list a "recommended" products, I'm sorry to disappoint you. That's not how we roll here at SmallNetBuilder. If you've read this far, then you have a pretty good lay of the NAS land and need to do your homework. Or you could just skip over to the Popular NAS page and see what other people are researching and buying.
The products below are representative of the various product types and are reasonable choices that general consumers with typical NAS needs should be happy with. I have also tested all of the products and have linked to their reviews for your reference.
Server Killers: We don't really review these. But rackmounts from NETGEAR, Thecus, QNAP, Synology and Iomega fall into this group.
I'm sorry that this article came out longer than I wanted. But I guess that's a reflection of the rich choices available in today's NASes. But no matter which selection method you use, you'll find our NAS Charts and NAS Finder to be an invaluable aids in your search. And, of course, our in-depth product reviews are also a great source to tap. So go forth, and get the perfect NAS for you!