I consider myself a driving/racing game fan, but I can be picky about what I enjoy. I really enjoyed Test Drive Unlimited (TDU), thought TDU2 had its moments, loved Dirt 2 but not 3 as much, loved Forza 3 but not 4 as much. That said, Forza Horizon hits a sweet spot that has me completely hooked to it.
The basic premise is an open world, with a good choice of cars. The cars are king in this game, with the focus clearly being on them, rather than any ridiculous side-story (I'm looking at you TDU2!). The big names including Lamborghini are here, except for Porsche, but having RUF will, to a certain extent, fill that niche. Within a brand are a wide range of models, with anything from an Aston Martin DB5 of yesteryear to a modern day One-77 to choose from.
For what I want from a racing game, the handling is perfect. I've heard dedicated racing gamers and journalists say that the handling can still feel a little simplified and not a 1-2-1 simulation of actual driving, but if you appreciate that it's close, and not trying to be a direct simulation, then it's close enough for most. If you're a beginner, you can have a multitude of assists on, while those more experienced can turn off your racing line, ABS, automatic gearbox, rewind features etc., and be rewarded for doing so by getting cash multipliers for winning races with these assists off. Cars have enough character to feel unique, and you'll definitely have to adjust your driving style from one car to the next. Just upgrading your car, using the typical suite of Forza tools, can drastically alter the feel of a car (as you'd expect).
The world itself is big but not huge. Don't go into this game expecting 2 islands to play with, but do expect to be able to get lost in the scenery through simply enjoying driving. The graphics (in my opinion) are gorgeous, with the graphics engine really bringing out the best in the cars and the scenery. The day night cycle brings with it some glorious sunsets, which you'll want to stop and admire, perhaps using the Photo Mode to capture the scene. The camera settings are also head and shoulders above TDU2's offering, with proper settings such as shutter speed, aperture, contrast, vignetting etc. that all come together to provide the tools for some great photos.
I'm not a multiplayer kind of person, so look to another review for that, but I've heard it's pretty good - however the AI of cars in the world when playing offline can also create ridiculous fun, with other drivers actually going for it. In TDU you'd find another car which would only cruise around at low speed unless you started a race. Not only are instant challenges much better in Horizon (only 1 button to press before you're racing) but you'll find cars will actually push themselves outside of races, and so can be good fun to follow and race for miles, not just pass and then beat to a marker.
All in all, this is a game that I had to buy having played the demo. Once the demo had its claws in me, I pre-ordered the game, and am so glad that I did. I'd recommend this game to anyone who likes open world driving games, or is looking for a driving game that can be serious when you want it to be, but on the surface is good fun.