See, that's the big thing - if you dish out $500.00 or more for installation and a HD/Tivo unit (that you are responsible for) then that is a very serious cost consideration. That's for ONE set! So, you pay about $46.00 a month for a single set plus Tivo fees ($10/mo), and you are responsbile for somewhat flakey (in my experience) and pricey hardware - do you doubt the hardware will be outdated in 3-4 years? Cable is just under $50.00 a month for a comparable channel lineup. It's $20.00 for a HD/DVR rental and that's it. One set then has your HD/DVR service. Over 4 years, the price between the two services is almost identical as long as your only upfront cost for DirecTV is $500.00... Is there still a $5.00 a month charge for additional DirecTV boxes for a second or third TV in the house? That'll completely change the cost of DirecTV when compared to cable. IE: My home has 8 locations to hook up cable, only one of which I care about having HD/DVR and digital quality, all of which I get for $70.00 a month - DirecTV would cost $90.00 a month for just the service (at $5.00 for each extra set).
The value of DirecTV just doesn't seem to compare, but I will not knock the quality of any of their channels which all still look really good. But, the constant questions exist about what local channels are available (Fox, UPN, NBC, ABC, CBS, etc.) what about local HD channels that are available? This just never seems very clear on their website.
I gotta scream out that cable is HIGHLY dependant on where you live. I live in the Washington, DC area which is one of the leading areas in the nation for cable service with HD broadcasts. So, while you can be pretty well assured that DirecTV will give you excellent service anywhere in the nation, cable is only as good as your local provider can give you. In the last 4-5 years, I have seen a shift in my area from high-end installations away from DirecTV and into cable as cable technology is finally matching that of DirecTV.
Oh... there's also CableCard and soon built in DVRs w/CableCard.... that'll be something.
Edit: Don't forget that you will probably want high speed internet, which quite often is discounted by cable if you also have cable TV service from them.