The TS-U200 offers an interesting mix of features at a price point within range of similar products. Its card reader functionality is well done (although it would be better if it could also automatically copy data from USB flash keys) and the "Download" feature is unique among all NAS devices I've tested.
But this little guy has downsides as well, with the security hole I turned up being the biggest "uh-oh". While casual users won't notice it (and I might not have, had I not been doing my usual poking around for ways to get at the TS-U200's Linux underbelly), you should not place the TS-U200 directly on the Internet or expose its FTP, Telnet or HTTP ports to the Net via router port forwarding until TRENDnet releases a fix.
However, what many users will notice is the product's slow file system performance - significantly slower than any other product in my 128MByte file transfer comparison. You might get by if you're using it for casual file serving, but if you'll be frequently moving large audio and video media files to and fro with it, you probably won't be happy.
Finally, if you're trying to decide between the Linksys NSLU2 and the TS-U200, I'd be remiss if I failed to mention another area where the NSLU2 bests the TS-U200. The NSLU2 boasts a large and active development community that works hard to extend the stock capabilities of the device far beyond its original scope. TRENDnet may have intended for the TS-U200 to compete with the NSLU2, but if so, they've fallen short of that goal.