I am looking for an affordable and usable NAS too, and came to the same conclusion: it should be the Synology DS211(+) or the QNAP TS-219P+. I think that choosing one of these will always be a good choice, because you already weeded out the mediocre ones (like LaCie 2big Network, WD MyBook World II, Iomega StorCenter ix2-200, Thecus N2200, etc). Most NAS's that are as good (or better) as the 211(+) and the 219P+ are more expensive (e.g. the Netgear's and Cisco/Linksys).
The Synology DS211+ and the QNAP TS-219P+ are almost identical in features and price (at least where I live). The 211+ has a SD card slot, so if you often transfer pictures from your camera to the NAS, this is useful. The TS-219P+ supports placing bigger hard disks and RAID level migration without shutting down or backing up the device. Useful if you want to start with 1 hard disk, want to start with small hard disks, are not sure which RAID-level fits you best or are just plain lazy (like me). If you look at performance test (like these:
http://www.smallnetbuilder.com/component/option,com_nas/Itemid,190), than it looks like Synologies are faster with writing large files (better for recording and saving audio or video), while the QNAP's are faster when writing lots of small files (better for backing up the contents of the hard disk of the computers on your network).
The 211(+) seems to be the more silent one, and the 219P+ seems to be lower on power consumption. But this also depends on the make and model of hard disks that you will install. The 211+ is the only one to support encryption of your disks (not important to me), but using this will decrease writing speed with 50 to 75% (Google for tests).
For me support of Apple features count, like iTunes, AFP and Time Machine. Until now QNAP was lacking Time Machine support, but its latest products (119P+, 219P+, etc) are now as fully featured as Synology NAS's. Maybe even more, because the 219P+ supports HFS+ (the Apple format for drives) on external drives, while the 211 and 211+ do not. Both companies have good product forums and support, I concluded from Googling.
If you want to pay less, but still own a fast NAS, the Synology DS211 (without the +) is the one. Certainly if you do not care about the SD card slot, eSATA support, or shutting the device down for hard disk related maintenance. I think I am going to purchase the 219P+, because I have the feeling it is a better "quality" product (as in more durable). But some will call it over-priced. I am still in doubt, because flipping through the manuals gives me the feeling that the 211(+) has many nice features that are more "hidden in the manual". So maybe after studying both manuals (available for download) I may conclude that the 211(+) is the best after all.
Bottom line: you should know what features are most appealing to you, and thus you still have to decide for yourself. To help you, you can find a lot of (links to) expert tests and reviews on
http://www.testfreaks.com/network-attached-storage/.
Good luck!
