Yeah I stream stuff, started out on BBC iPlayer with Heroes back when it first aired here (I saw half an episode on TV, and checked online to figure out I was a few weeks late into the series, and iPlayer had the series catch up for it) I don't stream PPV stuff though as it's mostly sport and new film releases.
Then I watched Misfits series 1&2 on 4oD
Then I watched the whole of Farscape on Megavideo (at the time it only let you play 72 mins before you had to wait an hour to play again, so I think I started swapping and changing with Gorillavid) and a year later the same thing with Stargate SG-1 (except I had stopped using Megavideo for the most part) Those were two shows that I had seen most of before on TV, but had not seen them in the proper order or consecutively due to TV being scheduled (which is the main reason I stream now)
Past couple of years I have caught up on a few TV shows I missed, mostly from early 2000s, and recently done the same with films too. I generally don't stream films made in the last couple of years (actually only two from 2012 so far, and one of them I didn't realise till after) I guess from some kind of guilt about not paying for something so new. TV shows though I will stream ones that currently air in America but not here as it can't be guaranteed the show will air here anytime soon if at all.
Unfortunately some link aggregate sites (Primewire) and streaming sites (the famous Megavideo of course) are being taken down or blocked by ISPs here now, and a lot of links lead to videos already taken down, which can be frustrating. Putlocker is generally the best service IMO, the video quality is good and it loads fast and buffers the whole video (unlike Youtube which stops if you pause the video, or iPlayer which only seems to buffer a couple of seconds ahead, so if you have a poor connection the video keeps stopping) and doesn't require you to hand over your details (or force Amazon cloudfront like 4oD now does) but I have had a lot of recent Putlocker links that are expired.
I don't know about other people but with me it's not like streaming for free has stopped me buying DVDs, in fact rather the opposite... I estimate my collection has almost tripled due primarily to streaming shows/films I might otherwise have never seen, and definately not bought.
I should actually be getting the DVD series of Wonderfalls for Christmas... which has finally released here in region 2 nine years after it aired (and eight years since the region 1 DVD)... Hmm, actually it apparently aired on Sky1 here back in 2005 (though of course not in the right order) but again, scheduled TV missed the first time that and is otherwise impossible to legally watch in this country until now. Thus streaming 