I think describing it as flawless is a bit much. It is a great improvement over the status quo. It allows you to put what you've learned to immediate practical use. It is a brilliant example of a satisfying non-monetary trade. It takes a desire for knowledge, provides that knowledge, and allows the recipient to "pay back" the favor by using that knowledge to assist Duolingo. It makes learning fun and enjoyable.

That said, there are glitches. It can be very inflexible at times. There's more than one way to skin a gato, and Duolingo can be very forceful on it's interpretation. I'm also not sure how useful it can be when it comes to learning a new language from scratch. I already had a few years of Spanish before I started, so it was a lot of fun brushing up. However, learning a new language without a human being there to help you and talk you through it can be rough. Duolingo tries to compensate through a forum/qa solution, but there's nothing like talking to a person in the flesh.

That said, it is miles and miles above stuff like Rosetta Stone, etc.