K-On! was about the "in betweens" (as the author/artist Fred Gallagher of Megatokyo puts it) - those moments in between the big stuff... or in this case often the practice itself or the big performances. When the series first aired, I was sort of hoping for a "song a week" like the Monkees did in their live-action series (new music video most episodes) -- but finally settled in especially after Azusa arrived and added some grit to those moments.
By the end of the franchise, I had indeed gotten quite a pile of songs from the series, from the character CDs, etc. so I can't really complain too much other than it would have been cool to see more of the songs animated.
I can't really dispute Reckoner's review because its his subjective assessment after watching the movie. I am, frankly, impressed he watched it
because there's no way I'd sit through a couple of hours of, say, Gundam Frolick or Naruto-tastic