This thread, I gather, is all about saving money. A cable DVR is probably around $20/month. Not sure what used Tivo's are going for, but a new Roamio is $600 plus $500 for lifetime for a total of $1100. A DVD/HDD recorder is around $250. A cable card tuner is around $100, assuming you can use it in a computer you already have. So the payback period on Tivo is 4.5 years, DVD recorder 1 year and cable card HTPC 5 months. Even if you have to spend another $300 for a brand new PC, bang for your buck is by far the biggest with HTPC. Since I've never seen anything On Demand worth watching, it's no big loss for me. I just record whatever I want to watch or stream it online. Once you have a DVR, your need for On Demand will likely drop to zero as well.
If you're willing to consider a different service, like satellite, you can save even more, especially in the first 2 years, during which your DVR is essentially free. Switch providers every 2 years, and you can keep the savings going indefinitely.
BTW, like a 6 tuner Tivo, any 6 tuner cable card tuner can get 6 channels with a single card as Kelson said. Just buy a single device with the number of tuners you need rather than multiple devices with only a few tuners.
In my system, I have a 4 tuner device in my PC that serves all 5 TV's (and several other PC's, laptops, tablets and phones) in my home for a single $2/month cable card fee. That would cost me $40-$50 if I leased my DVRs and boxes from the cable company. Tivo lifetimes for that many boxes would cost a fortune, with a payback many years down the road. So I couldn't even entertain that option at all.