I liked it, I liked it better than MIB 2, and somewhat better than MIB1. The jokes were clever, the acting was reasonable, and the story done about as well as any time travel storyline can be. Of course the entire time travel, change history thing is way overdone in science fiction these days, so I would say it loses points for that. Not as many as something more serious would, but I'm still tired of the whole time travel thing.
On the whole, I'm glad I waited until it came on Starz, since I get that for free with my employment, so I didn't actually have to pay to see it, but it wasn't a bad show, and while it probably wouldn't have been worth seeing in a full price theater, I'm not sorry I saw it. I'd give it about 4 stars out of 5.