Generally speaking, wireless media streaming devices are all unreliable. There are just too many factors that can influence things that can't be accounted for. For example, 802.11b networks probably don't have the necessary bandwidth to stream some higher resolution videos. A lot of hardware decoders for codecs like Xvid and Divx don't support Qpel features, since they require floating point operations to decode, and that sends the cost of the decoding chip skyrocketing.
For a little more work, and about the same cost as a lot of these wireless streaming devices, you can always set up an original Xbox console with the wonderful Xbox Media Center. The best method would be to have a modchip installed in the console, but it can be done with "softmods" as well which don't require cracking open the case. You then just attach a wireless bridge device to the ethernet port on the back of the Xbox, and you've got instant wireless. While Xbox Media Center is limited by the Xbox's 733MHz Celeron CPU and 64MB of RAM, it's probably far more reliable than any of those media streaming devices. If you install a modchip, you can also replace the hard drive in the Xbox with a much larger one, then simply copy the videos you want to watch to the Xbox's hard drive, and watch them from there. Xbox Media Center can also stream music files, as well as pictures, making it a fairly well rounded media hub. It even plays games to boot. If you don't already have an Xbox console, you can pick up a used one off ebay or at stores like EB Games and Gamestop. There are also solderless adapters for installing modchips if you're willing to pay a little more. Modchip prices can range anywhere from about $10 to around $100, depending on features. Just be aware that chipped boxes connected to Xbox Live tend to get banned.
Microsoft also has its own version of Xbox Media Center called something like Media Center Extender. It's not free, from all accounts it's a pale imitation of Xbox Media Center, and it only works with XP Media Center edition. The biggest thing it has going for it, is it will boot off an Xbox game CD without any modifications necessary to the Xbox console.
It's all a bit more work, might even cost a little more, and the Xbox console looks rather ugly, but it's probably the most comprehensive solution of anything else out there.