Book Rambling: DVD/Blu-Ray Question

I'm calling this a "Book Rambling" post because of the style it's ended up as, even though it's not about books at all.

When you're buying DVD's or Blu-Rays, especially for TV seasons, are they for shows you've already seen on TV? or do you buy for shows you haven't seen but think might be interesting? In other words, how much of a risk do you take?

I'm asking because I seem to find myself doing the latter, and sometimes it works out, while others not so much. I guess in part, I'm wondering how to judge a promising looking season/show, or maybe I'm just judging too soon.

For example, I bought a couple of seasons of Stargate SG-1 a few years ago, not having watched any of the series - maybe an episode or two but I'm not sure. I was hooked immediately on watching the first episode, Children of the Gods. That fascination kept up all the way through the next nine and a half seasons - though I found the last couple of seasons not quite as much to my tastes. I think it's because I liked the original four characters and their interactions - somehow the addition of Cam and Vala didn't work quite as well for me.

Based on the SG-1 seasons I'd seen by that point, it made sense to buy the Stargate Atlantis seasons unseeen - still haven't gotten around to watching them though. It's the same world and premise though, just slightly different characters, I think.

I've gotten a number of other seasons of things where I've not seen the show prior to getting the dvds as well, the latest being the new Amanda Tapping show, Sanctuary. I'll admit that I've only seen the first episode so far, but it's not grabbed me the way Stargate did. Maybe I'm expecting too much from the show - half the reason I bought it was because Amanda Tapping is in it, and it's completely a different type of show. I'm definitely planning to watch more though.

I've been looking at the Game of Thrones DVD's as well, but the price is definitely holding me back. Well, that and the fact that I haven't been able to read my way through the first book in the series as well. I get to chapter ten and stall out, both times I've tried to read the book now.

What are the criteria you use when buying seasons of TV series that you haven't seen? Or, do people still buy seasons of TV series rather than simply streaming them from somewhere like Netflix?