Netflix streaming has a lot of licensed content. It doesn't get most movies until they've been out on disc for awhile, though, and licensing prevents them from getting certain properties. For instance, nickjr was on there for years, then they didn't sign them again, so now they're not.
You can view netflix streaming using most internet connected devices. Lots of bluray players have it built in, roku, google TV, etc. Tablets, phones, computers. Chances are it'll work.
For TV they most frequently only get shows as complete seasons, so they can't be used to replace Hulu or your TV channel's streaming service.
They don't carry much, if any, sports.
The best way to think of netflix streaming is a way to access DVD and bluray available videos over the internet. If a show/sport/etc didn't show up on DVD in the market, it probably won't be streamed from netflix.
They do have good quality HD over decent internet connections.
Best thing to do is try a free trial. See how it fits, if at all, into your entertainment. We can almost always find something interesting to watch. They have a very large selection of Anime, foreign and independant films, along with Disney and major films. There will always be some films that won't be there, and lots of them will only be on there for short periods of time. For instance the avengers, first two iron man films, and thor are on there right now, primarily to promote, it seems, the newer films, but I don't expect them to stay on there for long after the next avenger's film comes out. Some will, but not all.
Netflix still has a DVD/Bluray by mail business, where their selection is much larger because they are not bound by licensing problems, but you can't get things instantly. They don't have discounts if you subscribe to both services, though, so you can see them as essentially separate services.
Netflix streaming only allows two streams at once. So while it probably isn't an issue for you now, we can have the kids watch something in the playroom, and we can watch something different elsewhere.
It's a nice service, but it's not the end-all-be-all for online entertainment.