I almost never have problems with Netflix on the Roku but I'd say the UI and lack of a keyboard and mouse make using it pretty unpleasant.
I use Netflix on Roku several times a week and don't have any issues with it. That said, it's probably because I don't realize how much better it looks/responds on other devices.
There's an iphone app that makes it better, but it works over the network. And when I say "network" I don't mean "your wifi" I mean "it connects to Roku's servers via your wifi so that every time you hit an arrow key it has to go to your hub, through your modem, up to Roku's servers, back down, through your modem, back to your hub, and to the Roku." Which means it's borderline useless at the best of times and completely fucked 90% of the time.