What makes a good summer movie? In my book, it's always meant a film that lets you escape the heat for 2 hours with a good action story and likeable characters (if you already have had some familiarity with them, it can mean a more enjoyable ride). MIB III has those basic qualities, and doesn't strive for any heights above them. No groundbreaking work here to be sure. It seems the producers sat down at the planning stage and said: "We should do a MIB sequel, since it's been awhile, and people have mostly forgotten about MIB II. We should make this one better." They succeeded their goals in coming up with a time travel story (it helped the Star Trek franchise move forward with ST IV). Josh Brolin steals the movie (and possibly future sequels?) from Tommy Lee Jones. Will Smith played Agent J with the same mix of humor and smarts that made him an appealing character in the first place. Throw in a really bad-ass villain, good effects, and snappy dialog. There's your recipe for an amusing diversion. The addition of AC on a hot day will only add to your enjoyment, unless of course you see it at a drive-in.