Pros: Fun gameplay, sim-style that isn't quite as technical as actual sim, interesting world, expansive free-roam area
Cons: Half the cars are DLC and cost extra, repetitive car-specific challenges, limited control config, no split-screen racing
The first Forza I bought and played through from start to finish was Forza 2, and it was a great game with tons of configurable options and a great selection of cars. It just didn't have as refined a physics engine as GT3. It was, however, a HUGE step up from PGR, so I was happy. Then Forza 3 came out and it improved the physics, increased the number of available cars, and refined the (very detailed and expansive) tuning options for each car. It became clear at that point, that this franchise was going to stick around for a while.
After playing through every...single...track, race, and tournament in F3, I was completely hooked. I don't even miss GT3 anymore. I have a nostalgic affinity for it that may never fade completely, but I now know that the Forza line is as good or better.
I got Horizon about two months ago, and have successfully unlocked all but one of the Barn-find cars, completed challenges on something like 50 cars, and have achieved the Orange wrist-band on the central 'story-mode' of the game. I play the game maybe once a week or less, but when I do, I'll do 3-4 cars worth of challenges, race other drivers in the freeroam world, and maybe if I'm extremely bored, I'll run through an actual race. It's a good time-waster, but it's just not as engaging as some of the other games in the series.
The tracks are just ok. My favorite aspect of sim-driving, drifting, works fine, but the cars tend to defy gravity and friction to some degree in order to force themselves back into a straight line when I get sideways. The 'trick' scoring is weird and just not very satisfying.
Overall, it's playable: the car selection is good, driving is fun, and free-roam is adequate...but it just doesn't excite me the way some of the other installments have in the past.