It's been many years since I've frequented the TIVO forums, so I'm a little rusty relative to exactly what the Series 2 can and can't do, I mainly remember being convinced that I wanted to stay with series I.
There are 2 aspects of what you can and can't do with the TIVOs, one aspect is whether it is functional without a subscription, and the 2nd aspect is whether you can hack it to add 3rd party software and hardware, and this is hacking in a good sense, getting the box to become more functional and to do things that it wasn't capable of doing without the hack, not an attempt to steal service. The series I TIVOs were very hackable. On my TIVOs, I installed network cards (sold as TIVONET and TURBONET cards by 9th-TEE), pulled the hard drive, and altered the Linux OS so that it would allow you to log on and use the TIVO and run programs. There were several programs people wrote for the TIVO OS. My favorites were TivoWeb and mfs-ftp. TivoWeb installs a web server on the TIVO, and it allows you to control the TIVO from a web page on your laptop computer instead of via the remote, and do a bunch of other things, like re-naming recordings (important, since without a subscription the recordings are just labeled by time/channel rather than program name). TivoWeb can even be used remotely from another location over the web, but some of the functionality, like the web based simulated remote control pretty much requires you to see the screen while clicking on things on your comuter.
The MFS-FTP program (and there are a couple of other similar programs) allow you to extract recordings from the TIVO, downloading them to your computer. The similar programs also included a video editing function, so you could remove commercials, etc, however I preferred the MFS-FTP program, as it would function with regular FTP programs on your computer. Once on the computer, the video was in a .ty format, which is similar to a .ts file, and can be converted to a .mpg file from the newer versions of VideoReDo, and I think VLC also played and converted them if I remember right. OR, with MFS-FTP, after being stored on your computer, you can keep them in the .ty format, and FTP them back to the TIVO for playing back there.
One other functionality that you get when you hack the TIVO is the capability of setting the TIVO's clock. A non-subscribed TIVO typically loses control of time after a while, which makes making scheduled recordings more difficult. You can connect the TIVO to a phone line and have it phone home, which will automatically set the clock, however for me this was a toll call. Also, if you want the TIVO to phone home to get updated firmware (TIVO allows series I TIVOs to update their firmware even if not subscribed), or if you want the TIVO to contact TIVO to get an updated list of your local OTA or cable or satellite channels (this is very important if you buy an EBAY TIVO, since it will probably come programmed with channels from another area that don't match your channels), this also requires a SLOW phone connection via dialup, which for me was quite expensive. Of course if you live in or near a big city, there are probably local phone numbers that the TIVO can call to perform these functions, but I remember that it cost me something like $40 in phone calls the first time I set up my first TIVO, and that alone convinced me to hack the thing. With the Turbonet card installed, you can have the TIVO phone home via the internet, which is much faster with no long distance fees if you're out in the sticks like me. OR, if you're familiar with Linux, you can log on, and change the date/time manually. It's probably possible to change the channel lineup manually too, but I never learned how to do that, so I just have it phone home via the internet when I need to do that.
All I described above, is what you can do with a series 1 TIVO, pretty much all with TIVO's knowledge without objection, although they don't officially approve of the extracting videos aspect. With TIVO versions after series 1, they started removing the capability of doing various things, and/or made it more difficult to hack the TIVOs without a subscription, up until the more recent TIVOs became complete doorstops without a subscription. As I said, I'm not positive which versions allow what now, but to be safe, a Series 1 is really what you want if you want full hackability. A series 1 TIVO will basically do everything it can do if subscribed that doesn't require use of the program guides that it downloads from Tivo central. Ie with a subscribed TIVO, the TIVO knows that say Leave it to Beaver is on at various times, and it will tend to automatically download that program if you've recorded it in the past, or you can tell it to automatically download all football games of a certain NFL team. You can't do that with an unsubscibed series 1 TIVO, but you can tell it to record for 4 hours every sunday afternoon, or at specific times. And the unsubscribed series 1 TIVO can pause, fast forward, slow motion, jump forward in 15 min increments through recordings, start recording a program from the beginning AFTER it has started, and other TIVO functions that don't involve the program guide. It's basically the program guide functions that you're subscribing to, not the TIVO's hardware's functionality.
One thing about ANY TIVO though, is that the hard drive is running constantly, because it is recording at all times, whether you are telling it to record or not. It generally has about a 1/2 hour buffer, which allows you to pause or back up while watching live TV. Since the hard drive is working so hard constantly, they generally have a lifetime of 2-3 years before they start failing. When a TIVO drive fails, you need to pull the drive, and copy the drive's contents over to another hard drive, and install that on the TIVO. Every one of my 3 TIVOs is on at least it's 3rd hard drive, although in most cases I changed hard drives in order to obtain more recording capacity. But it's really a good idea to get comfortable with Linux and the programs requirred to copy the TIVO's OS from one drive to another, and it's nice to make a backup copy of the OS before the drive fails. I'm now starting to worry that soon I may have problems replacing hard drives, because the TIVOs use IDE (or EIDE) drives, and now when I go to Staples, most of the drives I see there are SATA drives. Also, the drive upgrades work easiest if the drives are relatively small by today's standards. Most of my TIVOs have like 160GB drives (only using 128GB). You can put it bigger drives, but it's a bit more confusing, so I just keep things simple using the smaller drives. But it's getting harder and harder to find small IDE drives now, so it may get harder for me to upgrade next time my TIVO's drives fail.