The 721 has 2 satellite tuners built in, but only supports one TV. You could split the signal and support more TVs, but all TVs would be watching the same program.
I love my 721. In fact, I've loved all 6 of them that have passed thru my hands. They are GREAT when they work. But they do seem to have a large number of HD and power supply failures. It'll probably be useful for another 3-4 years, until Dish moves all programming to MPEG-4. The 721 does not support MPEG-4 or 8PSK.
As you can imagine, 721s can be scarce (discontinued long ago). Check the classifieds or eBay.
The 501 and 508 are single tuner DVRs and do NOT have a monthly DVR fee. The dual tuner 721 does not have a DVR fee either. They are grandfathered, because Charlie promised they would never charge a DVR fee on them. All units that came out after then have monthly DVR fees.
I have VCRs (dusty), DVD recorders, and stand alone DVRs. I also have the ATI All In Wonder HDTV card in my main PC. I gladly pay Dish a DVR fee on my ViP622, just for the ease and convenience of it's use. I don't like setting manual timers, synchronizing 2 pieces of equipment, or setting up IR blasters. Been there, done that, got the T-shirt - and burned it. Not worth the trouble.