These units do have their own threads, but oh well...
1) The tuner is about the same as the LG TV I have, which is the same as the old Zenith (owned by LG) DTV converter boxes. I've found these tuners to be far more reliable than say old Sony ones (from the mid 2000s), which failed to pull in the same stations that the LG tuners could.
2) The maximum drive size is 2 TB, but since the drive is swappable and formatted normally with NTFS (no asinine encryption like the DTVPal), you can have as many in-service HDDs as you wish, provided you don't wear out the SATA connector on the DVR. You can also just transfer your recordings to a USB HDD or NAS to keep space open on the internal drive for new recordings while maintaining access to your old ones over your home network.
3) It comes with a USB WiFi antenna.
4) An Internet connection is only useful for downloading the latest firmware (if you don't want to put it on the HDD yourself and do the update that way) and updating the clock via NTP. WAN connectivity is not required, although having the TViX on your LAN is useful for transferring files without having to physically connect a USB storage device. If you have no Internet access at all, you can set the clock manually or let PSIP do it (shudder).
5) You want M6620N+ for ATSC. The M664
0N+ is the DVB version.
6) The biggest user complaints are:
a) A red recording "dot" (affectionately termed "the meatball") stays on the screen the whole time a recording is in progress. The only way to not see it is to watch a program you're recording using the tuner in your TV. Plasma owners especially dislike this, since they don't want to burn the image onto their screens.
b) Whenever a scheduled recording is about to begin, you must tune away from whatever channel or recording you were watching and switch to the channel that is about to be recorded. After about 15 seconds, the recording dot will appear to indicate that the recording has started and you can tune away from it. If you select "Cancel" to not interrupt what you were watching, you're cancelling the timer and won't record the thing you had set to record at all. There is no way to start a recording without interrupting your current activity, due to hardware limitations.
c) ION stations that use the AirBox service (encrypted MPEG-4 channels) can cause the TViX to crash with their guide data. Although stations have rolled out a "fix" recently that decreases the likelihood of crashing, it's still not perfect. If you're affected by this problem, the only solution is to delete ION, qubo, and ION Life from your channel list. Thankfully ION has no worthwhile programming, so it's not a huge loss...