I enjoyed both from an entertainment stand point. In a lot of ways, even as much as I make fun of them, I think they are kind of over criticised. They are by no means good movies, but they both did a good job of appealing to their target audience.
What I find interesting about both of them, especially the first, is that they acted as a double edged sword for the import community.
On the one hand they brought a lot of commercial exposure to what was to a certain degree an underground community. This made big performance companies like Edelbrock, Holley, Summit, Jegs, etc. start to develop parts and focus their sales towards more than just cars with eight cylinders.
On the other we saw the sport compact community at least double in size, if not more, with a flood of "ricers" throwing fart cans and Type R stickers on every car that didn't have a V8. However, worse than that was the increase in attention from the boys in blue. As if there wasn't enough of that already. Many communities are quickly trying to put laws into effect limiting aftermarket exhaust systems, ride height, etc. I have commonly seen police ticket a sport compact for their exhaust noise while a Monte Carlo (or some such vehicle) rumbles by at a much higher db level with no notice.
Just some of my thoughts. None of the above factors is completely because of the F&F movies. But, I think both films still played a large part in the good and the bad.