The opener to this series has everything the film has, but obviously to a much lesser extent. The story opens the series wide arc telling of how the dragons from the film ('How To Train Your Dragon', as if you didn't know) adapt to Viking society and are accepted in Berk. The series fills the gap between the two films and answers questions you may have had while watching 'How To Train Your Dragon 2'.
The series has everything we loved about the the film(s). The characters are spot on and the relationship between Hiccup and Toothless is perfect. Aye, the animation isn't as amazing as the films, but is amazing for a TV series, the stories aren't as huge and epic as the films (90 mins>20 mins, obviously) and the series is aimed more at children than a family audience, but other than that it has everything the film has.
If you loved 'How To Train Your Dragon' and don't mind children's shows, you'll probably still love this. It opens up to a whole new, huge story that it untold in the films. The episode is entirely based on the creation of the Dragon Academy, opening up for an arc with many more entwined.
If the idea of a less mature and smaller story than the film episode excites you, I suggest that you give this a view. You'll probably enjoy it. Just don't go in expecting something massively exciting and gripping. Children who loved 'How To Train Your Dragon' will almost certainly love the series.