In my case, I bought my first TiVo (a Premiere), which I am using strictly as an HD STB (no interest in the DVR functionality, or any of the other features of the TiVo Service). Let me answer some questions from the cable-subscriber end of things.
1. Must you subscribe to the TiVo service to use the basic functionality of the Premiere? Surprisingly, no. The very reason I didn't is because it duplicates functionality accessible via my computer - if I want to get a recording of ANY show that I would normally watch, the odds are high to extremely-high that a torrent of the show is available, and of at least as high a quality as the Premiere's tuner+CableCARD combo. (Both PC and TV are 1080i-ready - therefore, the quality loss is none.)
2. The one thing you DO need (TiVo subscription or not) is a connection to the TiVo service - a broadband connection (wired OR wireless) is preferred. However, if your TV is NOT in the same room as your router, this can be problematical - especially since the TiVo wireless adapters are quite expensive. However, once you get the pairing done, here's how I tackled it:
A. I set up a staging area at my PC (so I could use its display, which supports HDMI, in addition to running a wired connection to the router). I went through the entirety of the Guided Setup (documentation is included with the Premiere) - remember, I had already gone through the pairing portion, so I was able to skip that section and continue with the rest of Guided Setup. The wired connection is only 100 mbps (signifcantly underwhelming for a gigabit router, which this is); however, it is still pretty darn spiffy. It asks a leading question about your cable provider and channel lineup (so it knows WHICH channel lineup to grab - some cable companies have different channel lineups by location) and asks about premium channels (so it can properly set the Guide to grab information about those channels), then it gets busy. The entirety of this portion takes maybe thirty minutes from beginning to end - the end is a reboot after the guide updates - taking you to TiVo Central.
Since I did NOT have an adequate length of cable to reach the Premiere at the staging area, I unplugged both power and Ethernet from the Premiere and moved it back to its destination.
B. Remember, I mentioned that I did not subscribe to the TiVo Service - nor do I have any intention to. Is my Premiere a brick? Hardly - I can still access the TiVo Guide (which looks like a cross between the existing Xfinity Guide and the TiVo Guide). It's at least as solid as any guide in existence.