The SA8300 is a box with great ideas, but with software problems which Cablevision and SA are trying to resolve.
If you have the IO service, you hook up your SA box with standard RG6 cable--one cable feeds both internal tuners. Hook up your TV by HDMI (or component if you have no HDMI/DVI on your TV).
The box can record up to two different channels at a time, and play back something previously recorded. There's a buffer for live TV watching, but there are some bugs (see below).
Data is recorded in direct digital format, as transmitted by Cablevision, so there is no further conversion (and degradation) due to saving it on disk.
The internal HD is 160 gigs, which can hold perhaps 20 hours of HD programming or 160 hours of SD programming. The box has an external SATA port, and many users have reported successfully connecting external 300 gig hard drives to increase their storage time--the system automatically recognizes the connected external drive, formats the disk, and seamlessly integrates the external storage room.
Downside: some users have complained of noise; the hard drive will start up at odd times for system maintenance.
Now to the software: the major bug affected Cablevision users currently is the loss of scheduled recordings. This is pretty significant, because if you can't rely upon the recorder to record, why get it? I suggest you look at the Cablevision newsgroup on Yahoo, and read the recent posts on SA8300 issues.
I use a Panasonic DMR-E85 to record my programs (SD only). I am toying with the idea of getting the SA8300, once they've ironed out the software glitches. Note that Cablevision supposedly plans to implement VOD (video on demand) for all channels, using their own servers. This will remove the requirement for hard drives in each set-top box, but the downside is that the user will NOT be able to fast-forward through commercials.