It was good and entertaining but a step down from the first one and from a number of MCU films actually. Still a solid sequel and an enjoyable one.
The plot had a deja vu going on and the novelty wasn't there this time but it was a bit darker, equally funny, exciting and moved at a good pace. The original had a simple plot and it worked fine since they gave time to character interactions. This time they have a simple plot again (and pretty similar at times) but they try to make it a bit more complicated and it ends up a bit overstuffed and sloppy. Still the character interactions are great.
On the other hand every character had a moment to shine, some secondary ones got a nice background and a deeper development and Whedon did a decent job holding all those characters and stories together. The action was really well made even though it wasn't as memorable as in the first. The Hulk vs Iron Man sequence was the best along with the circle one during the finale.
It definitely has some issues and i can see where the complains come from but it's a good time at the movies and a fun experience.