Struggles with the acting. Particularly Skye. I'd say it's more for Whedon fans than Marvel fans. Plus if you look at his history of tv work - Buffy started slow, as did Angel, Firefly and Dollhouse. It's worth watching for the dialogue. You're not going to get as much action as the films but you are going to get great overarching storylines and storytelling as well as awesome character development. It was cool to see Nick Fury!
I let my lad stay up to watch it on Sat night (he's 7 and a half) - he's a huge Marvel fan. It left him bored. He wanted to enjoy it, but left him flat. Although it's pitched at a higher age group, he's sharp as a button and knows all about SHIELD etc. Didn't ask him, but I think he'd have given it 5 out of 10 judged by his reaction!
It's early days. It was never going to be as action packed as Avengers. It's going to be very character based. A lot of focus on "The Team" and as they appear to be above the level of SHIELD previously seen or written about, it's not going to pay much attention to cannon. It's not perfect but has great potential!