I bought a used ReplayTV. I have the 4504; in my case, I got it at auction for a company that went under due to dot-com type fraud (it made big news here in SW Michigan last year, the SEC and the Fibbies came in, the CEO committed suicide, etc.). In my case, it appears I actually got the unit used in the CEO and his wife's personal residence.
Mine came with the lifetime subscription to the service (the cost is in the subscription, not the device), so I saved a bundle of money. The 45xx and 5xxx series models are the ones to get; the new 55xx series, like many TiVo's, are crippled due to the content-providing industry. Mine and the newer 5xxx series both have automatic commercial-skip, and the ability to share shows over the internet (caveat: you can only share shows with owners of the same series as you have). Mine works perfectly, and I'm very happy with it. Only thing I might do is eventually put a bigger hard disk in so that I can record more stuff. I dismantled my budget-HTPC because the ReplayTV works so well and is so convenient. Tons of A/V ins and outs, including VGA-out for higher end televisions.
ReplayTV is getting out of the hardware business, and going software-only, something you install on a PC with any Hauppauge WinTV PVR card. However, since the lifetime subscriptions follow the unit, not the user, you should be able to find a used 45xx or 5xxx model on Ebay. If you have any questions, ask them here.
P.S. Those "older Panasonic models" aka the Panasonic Showstopper, are early ReplayTV's.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890