I don't have the Roku, but am able to steam the movies via our Xbox 360 or the computer (which is where I mostly watch from, family watches more from the TV though).
I'm not quite sure what type of information you're looking for, but here is a site that lists all the movies that are available on instant watch. It's not operated by Netflix, but it's a good site.
http://instantwatcher.com/
Again, not run by Netflix, but here is a site that keeps up with films that are soon to be released on instant watch.
http://www.streamingsoon.com/
I would recommend Netflix to anyone. I have been very satisfied with them after starting a membership a few years ago. They also make it easy to report lost, or damaged videos (to replace if they are at fault, or for you to buy if you are and they take you at your word). Their billing dept. is top notch and all the info for canceling your membership is easy to access. That's not so with many other online companies.
My daughter has a classic film class in high school. She had to leave school today and returned later on, but missed out on the film, "Rebel Without a Cause." Well, she was able to go online tonight and watch it, so no staying over to catch up. It just really worked out quite nicely.
I have a 30 day free trial code if you're interested in giving it a shot. You can watch it online for 30 days, then decide if you want to keep it and buy the Roku (or other device). Cancel it before the 30 days is up and you won't be billed further. If you're interested, drop me a PM. Netflix is constantly trying to get you to get family/friends signed up, but gives nothing to the current customer if someone joins because of references. So, if you want a code, I have it, but I won't get anything from it (which is why I mention it because I wouldn't want anyone feeling any sense of obligation whatsoever).