Tivo is the best gadget that I've bought, own and use. If you watch any TV at all, it will change your life. I don't know what else to say about it other than it's the absolute best.
I own many Tivos now. I have:
The Directv Tivos are awesome because they have 2 tuners and they download the direct satellite signal so the picture is perfect. It also has much better guide data as it gets it from Directv and not it's own service.
Anyway, the following is probably the best place to learn about Tivo:
Update 3/20/04:
I went and bought the CacheCard from 9th Tee and installed it into my DirecTivo and I have to say that this thing is AWESOME! It's a combination of adding a ethernet port/connection to my Dtivo as well as adding up to 512mb of PC133 SDRAM memory for the tivo to use instead of a disk cache resulting in much faster menus.
I've got it so that I can control my tivo from my computer via TivoWeb+ and that I can extract recordings onto my hard drive via TyTools. More info at the forums at DealDatabase.