TiVo will change your life... for the better. Plain and simple: record shows by telling TiVo to record them (not when, what channel, how long, etc.), record all new episodes of X, record all movies w/ so-and-so in them, etc.
High Points:
Most people know the basics of the concept. Here's the low-down on the things I'm asked most...
1. You can record one thing and watch something else already recorded.
2. You can get a dual tuner TiVo, but you'll need two SEPARATE feeds to the TiVo (i.e., two lines from your satellite dish). Then you can record 2 things at once or watch one/record another. These are more expensive, but you can get a DirecTiVo w/ dual tuner cheaper and then the monthly TiVo fee is about half normal.
3. Pause live TV - if you don't realize the value of this, I can't explain it. :)
TiVo has many different versions which all some slight variations. If you want to go beyond 'average guy' level, check out this forum.
Comparison to ReplayTV:
TiVo seems to have a better interface from what I've seen and what others have said. It's easier to use. However, ReplayTV is more easily hackable - you can access it from the web and the PC easier to convert/archive shows, etc. I have a DVD recorder for this which is another post for another time.
Both can be made to do mostly the same things, but you get the idea.
My Suggestion
I recommend the DirecTiVo Hughes HDVR2 (dual tuner) w/ the Triple LNB dish (this is the oval dish w/ 4 lines from it and it's HDTV ready). It's got the satellite converter box built in so it's only one box to deal w/, the TiVo monthly fee is less and it's dual tuner.
This model isn't as hackable as some TiVo's, but it can do some and if you want to go beyond the basics than you probably haven't read this far.
Sites:
http://www.dvrinfospot.com
http://www.TiVo.com
http://www.TiVoCommunity.com
http://www.WeaKnees.com
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