I've never been one to go with proprietary DVRs as they used to be generally limited on hard drive space. 80GB was the norm, but now I'm seeing around 500GB. Yes, that may seem like a lot of room, but I (and my cable subscription) can fill that up easily. If it does fill up, then you have no choice but to delete some recordings as you cannot move those files from the DVR to a computer for example.
My recommendation would be to build a Home Theater PC that is used strictly for media viewing/recording. Hard disk space is essentially unlimited as you can simply add/replace media drives. The media format is not proprietary so you can record it on one system and view it on another.
Yes, there is a higher upfront cost, but I think it pays...