The usage isn't as hard as it seems. Two things kill hard drives - on/off cycles when the disk STOPS spinning, and excessive head seek when collecting fragmented files or small files. Since TIVO use is essentially non-stop tape recording of massive files, there isn't anything like the degree of fragmentation as you get with a PC.

If you save a lot of old stuff, and try to record around it, you will have more problems, but if you use the recorder as a time machine to give you access to standard programming at a convenient time, and then overwrite it, you'll be using the HD in just about the most efficient manner possible...