Well, the only stuff I tape is the prime time stuff. One or two shows a night, it's not too bad. I delete most of it when I'm done, so it stays around 40-50% full most of the time.
I used to have just a normal TiVo. It was great, but couldn't tape the HD stuff.
So when I got a HD DVR, I thought it would replace the TiVo, so I moved him into the study.
After discovering that the only thing the HD box could do well was tape a few shows and play them back, (user unfriendly as hell) I realized I still could use the TiVo for everything else.
By that time, I had gotten really attached to having a TiVo in my study.. So I broke down and got a second one to put in the living room.
That opened up the possibility of networking them together, and sharing shows between them. That part rocks. I also upgraded their hard drives. Added a 120 gb drive to each one...
My next step is to add FTP & Telnet to them. That lets you pull the video files out onto the PC.