Let me start by saying I was NOT looking for a Windows Media Center Extender, I was looking for a device that could handle my library of stored media (movies and music), especially it's ability to play DVD images, including the disc menus. For this purpose (provided you have a wireless N or wired network connection) the EVA-9150 is an awesome device, handling virtually any file format you can throw at it without problems. It's also a great internet radio streaming device, and has a nifty RSS feed reading applet.
The device includes a 500GB hard drive, but unless your home network is slow and you're having buffering problems, I doubt it would be of much use. If it is, you can swap it out for drives of up to 1TB in size.
Output options are plentiful, including HDMI, component and digital outputs, as well as standard composite and analog audio, so interfacing to virtually any home theater system is realatively easy. Wireless and wired networking are built in and easy to use, and software is included to help you catalog the media on the various devices you may have around your house. Overall, I found everything relatively easy to use for a person familar with PCs and home theater, though the sheer number of options may be intimidating to a novice.
The one place where there's room for improvement is it's ability to play streaming video from the Internet. The two biggest omissions are it's inability to stream video from providers such as Amazon and Netflix, and the inability to play Flash videos (such as those provided on Hulu).
If Netgear adds these capabilities to their next upgrade, they'd have a device that would be hard to beat. For now, they merely have the best way I've found to make media stored on PCs and network attached storage devices playable on your home theater system.