We've been Roku users for about 4 years now, but didn't cut the cord on our satellite service until the end of last year. It's only been about 4 months, but I can say that I don't miss it at all (okay, I do miss HBO a little bit, but I can live without it if it means not having the $110 satellite bill every month). We have Prime, Netflix, and Hulu Plus and between all of those, there's a lot to watch. All of the AMC shows usually hit Netflix streaming within 6 months or so, but if you can't wait, you can buy them from Amazon on Demand the day after they premier on AMC for $2.99 HD or $1.99 SD, plus they give you a discount if you subscribe to the entire season.
As far as quality, it's HD most of the time, but it can lose some quality or make you wait for it to re-buffer if your internet connection isn't very fast or bandwidth get's compromised from simultaneous use. I have a 7.5Mbs DSL connection and it does suffer from some of those aforementioned issues every now and then, but most of the time it streams in HD with no issues once the buffer fills, which can take a couple of minutes after you begin watching a program. If you have a house full of people all streaming independently, that will affect bandwidth and slow things down a bit. Overall, though, I'm happy with our Rokus (we have 3), especially considering the arse-load of money I'm no longer spending on the satellite service.