I have a couple TIVOs, and know nothing about Replay TV. Both my TIVOs are networked, and both have disks upgraded, which to answer a question above, is not hard, provided that you can follow directions, and are not afraid of pulling the hard drive and putting it into a PC to transfer the software. Normal off the shelf hard drives are used.
My reason at the time for going TIVO was because you could use it as a VCR if you didn't get the monthly service, and that it runs on linux OS so you can use it as a computer, and program it yourself to some extent, and for that reason, I went with the older stand-alone series one models, which can be purchased refurbished for around $100, although adding a network card brings that price up quite a bit.
I am interested in learning whether Replay TV can be used without a subscription? If not, that is a definate plus for the TIVO. If it can, how convenient is it to schedule manual recordings?
It is possible to archive TIVO recordings via the network connection, assuming you have telnet access, which is easy on the old models and more difficult on the newer models. However, the format you extract to, is quite difficult to burn to a dvd. I have made a few successful burns to DVD, but have more failures. Supposedly the DTV-TIVOs are easier to burn to DVD, because they store directly in the digital format from the DTV, which is both slightly higer quality than that of a stand alone tivo, but also in a format easier to convert to DVD.
However, if it is only archiving you want, and not playability on a DVD player, there are programs to allow you to re-insert the raw TIVO format back onto the TIVO, so you can store it on a DATA DVD or hard drive on the PC, then load it back onto the TIVO when you need to play it. From what I read in this forum, it sounds like the Replay TV might be more convenient for archiving, but it can be done on a TIVO that has been hacked for telnet access.
Re quality, in high quality mode, TIVO is better than any normal TV, and you don't see any digital artifacts even with very fast motion, except in extremely rare situations, so you won't see the difference unless you have a HDTV or big screen TV. In the low quality mode, there is definately a lack of quality, not in resolution or sharpness, but you start to see digital artifacts with fast moving scenes. You can hack the TIVO to use resolutions of your choice for the different quality levels, but I just use the high quality for everything.
My feeling is that people who start with Replay probably like Replay better, and people who start with TIVO like TIVO better. I really like my TIVOs, and unless the REPLAY could be used without a subscription, I wouldn't consider switching. I should also mention the TIVOWEB program, which lets you control the TIVO from a laptop, or even over the internet, which is very handy. I'm not sure if there is a Replay version of that or not.
BTW, I've also recently picked up one of those RCA 7000 recorders, and am not at all impressed with it so far. I love my TIVO, but am interested in learning more about the Replay.