The series starts out pretty terrible. If you're not already a fan of producer Joss Whedon, this show won't win you over. And if you are a fan, you'll spend most of the first episode squirming with discomfort, thinking about how you'd already talked up the show to your friends as something that "can't help but be good, because it's Joss Whedon!" You'll be treated to occasional great lines of dialogue wasted on a weak plot & a parade of smug two-dimensional cringe-worthy characters.
Yes, the first episode is disappointingly weak. But any hopes you have that the season premiere was an anomaly--a low point of the show born from the network's need for a quick pilot--will be dashed through the next couple of episodes. The show quickly plummets from weak to downright awful, and you'll spend a large amount of time each episode wondering why you're not turning the show off...assuming you haven't already done so.
But if you stick with it, the show does eventually get better. No, the characters don't really become more interesting...their personalities were locked down early on. But about 2/3 of the way through the season, a sudden plot twist connected to the movie Captain America: the Winter Soldier shakes things up. The overarching story becomes substantially more interesting.
The season ultimately closes pretty strong. It doesn't fully redeem itself, but it comes close. Most importantly, you'll hate the characters less.
In the end, this is not a good series. I could never recommend it to someone who isn't already a Marvel movie or Joss Whedon fan. But if you are, you're eventually going to give in, buy it & watch the entire thing, despite all the negative fan reaction you've no doubt heard.
If you do, I can tell you this: you won't hate it when it's over.