What success though? It's been losing Nintendo money for years, hell the 3DS is the reason why Nintendo lost money for the first time in 100 years. It also failed to do what the previous handhelds used to do and stop their consoles from bleeding money. Personally, I consider it as successful as the n64. If you wanna call it a "success" then the 3DS is a success. I personally don't.
I'd call it a success because 50 million is a good number and its had some excellent games released on it, which have sold well.