This is a game clearly built on a foundation of entertainment. Everything else comes second. Realism, customisation, and physics are all there in varying degrees, but if they didn't enhance the sheer fun to be had then they'd be out on their arses. As far as I can think of, everything in this game is fun - powerslides, huge jumps, nitro, using your opponents as bumpers, driving said opponents into posts and watching the driver catapult through the windscreen.... thanks to the damage modelling and somewhat over-the-top physics, it's even fun when you crash yourself!
Put it this way, if an official Dukes of Hazzard game hadn't been released a couple of months before FlatOut was, this would be the closest you could get.
It would be mindless fun except for the fact that it's not actually hugely easy! The game gets reasonably hard as you go on, but never gets frustrating. Even replaying the same track over and over again remains entertaining. The minigames are fun, and I do find myself coming back to them - they're not entirely throwaway. The demolition derby mode in particular is a good challenge, and (for me at least) endlessly replayable.
The main thing weighing in against the game however is the (IMO) terrible soundtrack. It's a completely mediocre succession of generic nu-metal/punk/hardcore, which sounds like it's been bought in some kind of economy bulk licensing pack (that's not a joke, I really think they've spent as little as they can!). There are a couple of ok tunes in there, but largely it's grating, nondescript guitar chugging and angsty nasal vocals.
Overall however, a great game with zero depth but loads of longevity and fun to be had!