Much much better than expected.
It had heart, I liked most of the jokes (not all) and I actually liked that J displayed real character here, contrary to the other 2 movies. Brolin was fantastic, Griffin was a touching and loveable character. They not only took the time travel concept that BTTF introduced, they took it a step further and the time travelling sequence was one of the best and smartest in cinema history, imo. Boy, does TLJ look older than 66!!! Will Smith's still got it, though, and his usual acting style was actually much more appropriate here, because of the 60s thing. Great cameos, too. Sonnenfeld was bringing it, he used every single cheesy element and line to the film's advantage. And Jermaine as Borris was great, a good character. Wished he was in it a bit more, though.
On the downside, you could tell the script was unfinished. A certain female character was wasted and the resolution, while great conceptually, fell kinda flat, although it had so much heart that I gladly gave it a pass.
A solid 7/10. Good job.