My experience (6 PVR's since 1999) is that you can expect a PVR to last for 3 years, and anything else is gravy, or at least by that time it's very obsolete (and the hard drive becomes iffy) and you will want a new one anyway. In 3 years HD content will be common, also, pushing the obsolescence issue.
At $5 a month you can then add about $180 to that off-the-shelf price for the 510. More, if you expect to keep it longer than 3 years.
I would consider the new $299 721 offer...the 721's software is now mature and stable. It has a few minor issues, but to me its a true Tivo killer, and has no VOD fee. Name-based recording is only a few months away (they say).
And with the 721 you can record two shows at once... so you can watch your show while her's records to watch it later (or the other way around in my house). PIP, 2 recordings at once... better interface...