The M-6600N is the only one of the bunch that's also a Personal Video Recorder, but we let it in this group test because we consider it to primarily be a media player.
It takes a 3.5in SATA hard drive, plus it features a SD/MMC/MS card reader and USB ports (both host and client, so it can act as a USB external drive). It lacks advanced features such as eSATA and component video, plus 802.11n Wi-Fi only works in 2.4GHz mode, not 5GHz.
When it comes to networking, the M-6600N supports Samba (Windows), NFS (Linux) and UPnP, as well as FTP access. While it offers more features than most, it's hampered by an overly complicated menu system. It offers 32x fast forward speeds but no video progress bar.
Networking is a little temperamental and it tends to get bogged down when accessing Windows 7 boxes via UPnP. It also handles YouTube, Picasa and RSS feeds - although the YouTube interface is cumbersome and files refuse to play.
In terms of formats the M-6600N is also found wanting. It sees PNG but won't display it, plus it refused to play MPEG-4 AVC files using Ogg Vorbis audio encoding. It also refused to play Windows 7 recordings (via UPnP). It doesn't handle ripped Blu-ray and refused to play ripped DVDs via Samba from our 10/100 network drive (handling ISO files but not VOBs). It refused to even see the Samba network shares on our gigabit Windows box, under Vista or Windows 7. Ripped DVDs will play from the internal drive, allowing you to access the menus.
Dvico's M-6600N wants desperately to be the love child of a TiVo and an Apple TV, but in practice it's far from a device that "just works".