WOW! What an awesome 3 hour epic!
1. The first section is a slow burn of course because scene setting needs to take place and the regrouping of the Avengers needs time after their massive defeat.
2. The second section with all it's time travelling is a nostalgia blast without looking like a cheapskate cop out. Most impressive are the quests for the space, mind and power stones. Most saddening is the soul stone quest.
3. The third section is an awesome battle at the end. Yes it would've helped if there were cameos from other characters (e.g. Daredevil, The Punisher and even Venom) but that's just a small gripe. I did shed a tear over Tony's death and heard two people sniffling in the cinema (Peter Quill, you've got Tony's blood on your hands you tantrum throwing brat).
In the lead up to this film, many fans were speculating that the time stone, Ant Man or both hold the key to undoing the damage. So it's a smart and astute move by Marvel to have Tony Stark (and an M.C. Escher work) outwiting fanspectations.
I'd like to end by saying that Infinity War is the better movie for those looking for a start to finish action fest.