Not much info out there on this device, especially from a Mac user's perspective, so here goes. I've used this thing for about 4 months now so this “review” might have some points that some others might miss. I usually just skip to the conclusion, so:

The Good

• Really does support a wide range of video formats. Haven't run into a file that it couldn't handle. They're of course out there (QuickTime comes to mind), but for all intents and purposes, I'd say the format support is more than adequate. Media stays in sync, well, even with network dropouts.
• Can seek. Well. Something that not every media player can do, especially over a network. And this isn't your typical “can fast forward but not resume playing”-type seeking either - push a button and it will just go faster, push a button and it will play. Nice and accurate.
• Solid subtitle support. Supports pretty much any format and comes with a custom font with a black border that looks crisp on any background.
• Can play back from any SMB or NFS share, comes with a built-in FTP server. Doesn't need any proprietary server software.
• Looks cool. The “brushed metal” faceplate blends well with any darker AV setup.

The Bad

• FTP server is ridiculously slow, averaging about 900kbit/s uploads! This means it will take over an hour to transfer a DVD image, onto the SATA drive. Very frustrating to have something on your LAN that's slower than your Internet connection :(
• SMB client is practically unusable. Your typical 1mbit/s DVD rip will stutter to the point of being unwatchable. NFS works much better, luckily.
• The copy function is nice (i.e. using it to pull stuff over NFS, instead of the anaemic FTP server), but doesn't handle folders! Also if there's a name clash between the source and destination, the whole operation is cancelled. These three problems make it painfully difficult to actually get media onto the device, especially since
• … there're no ports on the front. This one's of course tricky, since it affects the design, but it's just incredibly frustrating having to crawl behind your AV system to plug in a memory stick after you've realised it's going to take an eternity to get that DVD rip onto it over the network.
• There's no format command for the SATA drive. You have to format it yourself before installation. So while it's really easy to physically install a drive yourself, actually using it is much trickier. This means you're basically screwed if you only have a Mac laptop. The manufacturer recommends using NTFS and writing that requires the Paragon fs plugin…
• A bit noisy. It's not a G4 XServe, but slightly noisier than a Mac mini, IMHO. This wouldn't be a problem in an office environment, but in a living room, during a quiet scene in movie it becomes an issue. It also forces you to turn it off rather than use standby since it's startup time is pretty long. The HD that I use is a 500GB WD “Green Power” which should be one of the quietest 3.5” drives around (AFAIK). There's a setting for the fan speed, but it doesn't seem to do anything.

The Useless

• The remote. Why on earth does a device, 98% usage of which can be limited to roughly 7 buttons (power, play/pause, FF/RW, arrow keys) have 43 buttons on the remote?
• The front LED. This would almost go under “The Bad” - there's a button panel on the front under which they've put a bright blue LED. Having this thing under your TV is quite distracting and I can't understand why they didn't put an option in there to turn it off!
• AVCHD, bitmap and audio playback. What for? Why would I not just connect my camcorder, camera, iPod, whatever straight to the TV, amp, etc? Especially since this thing doesn't have any ports in the front…

The bottom line

The M-6500A has more than it's fair share of flaws, but it does fullfill it's most important task - playing back any material in your media collection - really well. The subtitle support is solid and it doesn't make you afraid to use the fast forward button. Too bad that getting the media onto it is way too painful. You almost have to wonder if installing a hard drive is worth it, considering how much easier it is to just stream over NFS. This is a shame, because with a large enough drive, this thing could make a nice little jukebox.