I've got a replay TV and I love it. I can record whatever I want and play it back instantly when I want to. Plus, the continuous record function is awesome. Right now, I'm going to tape Temptation Island and watch it tomorrow when I have more time because I'm going to frag now.
If you want to get a Replay TV you've got to do it fast. They stopped making the units and stock is running out. Replay TV is good in that you don't pay a monthly service charge. The Replay is also better than the Panasonic Replay because it doesn't have Macrovision copy protection built into it. Plus, there are ways to hack into it so that you can add more hard drive space.
I think the best feature of all is the TV guide. I don't have satellite so I have to either check the TV Guide or go to channel 99 and wait for all the channels to go by. With the built in TV guide you can scan out for days and see whats up.