I have been researching this a lot lately too.
I'm planning on getting one soon, and already have two 80G hard drives in hot standby.
From what I've seen the Replay looks like it has some really cool features too,
Like being able to go to a website from anywhere (work or out of town) and add a show to record to your replay.
Pretty cool huh?
I guess the older replay's had the feature too, but because they were modem and only updated once a day, you had to send it a day or two ahead of time.
What I see as the Cons of Replay is:
1. they don't have a DirecTV version at all
2. there is only one tuner.
I do think there is a IR interface to change the channel on your satelite box, but if you only have 1 box you can't watch something differant than you are recording.

With only one tuner, can you watch one recorded show and play another?
My guess is you could, cause you only need the tuner for the live show. But you still cant watch live and record at the same time.
I would love to get a HDTV box for my baby big screen 43", but its not worth it for 500 to 700 bucks for the half dozen channels.
There are only two models of DirecTV with HDTV decoders, but none have PVR feature yet.
For it to be the PenUltimate box it needs Dual tuner (at least) Digital Satelite ready, Digital cable ready, and HDTV outputs.
As of now the record quality is about half of HDTV, 500 lines of resolution if I recall the marketing jumbo, as compared to VCR at about 250. But I think thats pretty close to the quality of broadcast/analog cable TV.
For anyone using DirecTivo ......
How does the recorded image quality compare to live image quality?
How does live (directv) image quality compare to analog cable? and digital cable?