I've had tivos for years. We just got an HD TV and decided HD tivo was too pricey, so we went with the generic Time Warner DVR. It sucks. It sucks so hard we are thinking about spending the 800$ on the HD tivo. Here's why:
the DVR randomly erases things without warning, even when it seems like there is plenty of space. Also, it doesn't always erase the oldest program, I cant figure out how it makes its choices.
The schedule only has 2 or 3 days worth of data, so it's hard to choose things in advance
It's hard to search for programs (the tivo interface is much easier to use when you want to just browse catagories)
The fast forward doesn't have the nice little noises so you know what speed you are at
The fast forward doesn't seem to correct for reaction time as well as tivo does
It's not as warm and fuzzy overall.
Some of these things might seem trivial, but after becoming used to tivo, the time warner DVR is like Hydrox compared to Oreos.