I really liked the first movie. The second wasn't as great as the first one, but it was still entertaining and fun. Now we got a third movie and unfortunately it's like James Gunn and company have run out of steam.
We get a lot of backstory for Rocket and we learn how he became the badass killer racoon he is now. These parts of the movie work really well. The rest of the movie is however mostly the usual Marvel bang, boom, crash, explosions and massive fight scenes. It gets more and more boring for each Marvel movie, and it doesn't make it better that many of the scenes were clearly constructed to please people wearing idiotic 3D glasses. These scenes are really boring in 2D.
The humor often falls flat. It's like they have run out of great jokes for this lovely gang of space weirdos. Surely you can find something to smile at here and there, but compared to the first 2 movies it just doesn't work very well.
The PG13 rating doesn't make it any better. James Gunn is a lot more fun when he can go all in on violence and swearing, like the did in the excellent "Peacemaker"  series and "The Suicide Squad"  movie. If it wasn't bad enough with the lame humor, then just add a bunch of stupid kids too. There shouldn't be kids in action movies. It will ruin any action movie.
If you wonder how the hell Gamora can play a big part of this movie, because wasn't she dead or something? Well, yes and no. Read all about it here...
I'd recommend to wait for this movie to show up on some streaming platform. It's not really worth it paying half a billion to watch it in the cinema, unless you are a total Marvel fan boy or girl. Keep in mind that it is running for 2½ hours, which is much too long for this movie, considering that only about 1 hour of it is great.