If you're on Comcast's lowest tier TV-included package - "Internet Plus" - HBO is a free add-on. Right now we're paying ~ $70/month total for internet plus Cable TV (The TV channels include HD and are basically a throw-in, it's how Comcast tries to hide how many of its customers don't want cable TV anymore). I can't imagine paying $15 for any single channel.
The way I see it (as an Australian that moved the US ~18 months ago and is agog at the variety of entertainment options), Netflix is awesome because it's a nice cheap catalogue of mostly older content that I either haven't seen or am happy to watch again.
HBO for $15/mo seems reasonable to me because they have a (much smaller) amount of really amazing content (The Wire, Rome, Sopranos, Deadwood, etc) which I'll happily watch again and again, but they're almost always running the New Hotness (Game of Thrones at the moment, which I actually don't really care about that much).
The one channel of HBO for me is more value than many, many of the other available channels here put together. I have little or no interest in most of the other channels - almost everything I want to watch is on Netflix or HBO.
But, of course, YMMV depending on taste. I've been looking forward to getting HBO on a non-Apple device to try the trial and see how much of it I watch. I might end up deciding $15/mo is too much 12 months of the year but I'll definitely roll it out a couple of times and binge.