I have a friend that got a ReplayTV back in 2000 when I got my first TiVo and he's always insisted he got the better machine. Older Replay units (specifically the 4xxx and 5xxx series) are almost infinitely hackable, and a software package like DVArchive has a mindblowing list of features:
# See what shows your ReplayTV(s) have recorded right from your computer
# Automatically finds your ReplayTVs
# Download shows from your ReplayTV to your computer to watch, archive, burn to DVD/VCD, etc
# Automatic support for Replay 4xxx and 5xxx Series units
# Turn your computer into a virtually unlimited capacity ReplayTV show archive
# Schedule automatic downloads of shows from your ReplayTV to your computer
# Schedule automatic purging/deleting of shows you don't like but which your ReplayTV keeps recording
# Watch previously downloaded shows OR watch shows directly from your ReplayTV without having to download them first
# Control your ReplayTV remotely (turn it on, off, delete shows, etc)
# Manage your ReplayTVs photos
# Control your ReplayTV from your computer
# View and Search TV Listings
# Schedule new recordings on your ReplayTV (ReplayTV 5xxx series only)
# View lists up the upcoming recordings (and conflicts) for your ReplayTVs
# Access these listings, schedule recordings, schedule downloads, etc via a web browser from anywhere in the world
# Has a clean interface that allows you to see and use all your ReplayTVs from one place
The only TiVos I know that are capable of this entire featureset are what I would dub "ultrahacked" and are owned by electrical engineers and veteran software developers. But with ReplayTV and DVArchive, this kind of uber-functionality is as easy as installing a simple to use Windows program.
They've just released the 3.0 version, if you've got a ReplayTV and a home network run, don't walk, to get this package.