tl;dr If you liked Thor or Iron Man, watch it. If you didn't, don't. This a cheesy and earnest action film and there's lots of fun to be had.
I don't like Captain America. Stupid concept, stupid costume and utterly bland character. He's never been a character that has given me reason to care, nor one who really seems to have a USP beyond empty-minded nationalism. Marvel, however, have been taking properties that I either dislike or don't care about - Iron Man, Thor and most prominently X-Men - and making surprisingly solid and entertaining movies. The other pre-Avengers movies - Iron Man and Thor - had shown both a consistent tone and upwards trend in quality, so I found myself going into a movie starring Captain America with fairly high expectations. The movie did not disappoint, and proved that there's nothing particularly wrong with the character, so long as the execution is good.
Meet Steve Rogers (a man with such a singularly forgettable name that I had to Wiki it three time for this review): he wants to join the army, but as an asthmatic man of noticeably minature physical presence he is laughed out of every botched attempt to join. He continues to try however, earnestly fesiring a chance to stand-up to the cruel and power-abusers of the world. Such pure moral sentiment doesn't come across as obnoxious or annoying because of his innocence and his weak physical state. One day his desire to do good is noticed and he is offered a chance to serve his country, undertaking experimental treatment to make him a super soldier. Thus, Captain America is born.
What makes this hero - a bastion of compassion and heroicness as well as near physical perfection - continue to avoid being an obnoxious annoyance is the fact that he then goes on to become a piece of propoganda. Even though he is now a one-man army, he's still failing to fight and lay down his life as he desires. Thus, when the inevitable rise to action occurs, our hero has really earned his right to kick ass. The impossible odds the hero overcomes, his amazing physical prowess: we don;y begrudge him any of it.
The movie takes it's own ridiculous premise seriously, and through this we can also take the premise seriously. None of this "that mask is stupid" Green Lantern stuff, pointers have been taken from Thor in this one. This movie thinks it's as genuine a triumphant and heroic story as any, and such is it's earnest enthuaism that it is hard not to get caught up. This is pulpy WWII action, full of comic book silliness and war film bravado - making it a dramatic fare that people would take seriously was always going to be a big ask.
There is nothing here that will challenge or provoke, but there's buckets that will entertain. Captain America takes characters and plots at their most basic and does them right. It looks good, is paced well and it's certainly not lacking in cleverness.