Forza Horizon 1, on the X360, is an amazing open-world racer. It has, to my knowledge after over 100 hours into it, no deficiencies at all. It runs at a locked 30 fps with no screen tear and no stutter (ever). It looks amazing. I won't even qualify it by adding "for the hardware". It features a terrific physics model, and everything behaves as it should. It is the most immersive and addictive racing experience so far for me.
Cut to the sequel on much more powerful hardware. It stutters, though only occasionally. This can be minimized by disabling the DVR feature, but it doesn't cure it. It has a lot of noticeable pop-in detail. These are disappointing, but not fatal. What does make it almost unplayable is a shocking bug in the audio that freezes the pitch of the sound of the engine for one or more seconds. Any racer who isn't deaf uses the sound of the engine as crucial feedback to determine what to do moment by moment in a race (real life or simulation). That frees him to look out the windshield instead of down at the gauges. It becomes instinctive, and is absolutely essential.
Until they fix this (assuming they will), I recommend staying away. If you have no interest, but know someone who does, pass the word.
Here is the thread on the issue at the official forums.
To dispel any notion of workarounds, this happens online or offline, with or without DVR functionality, with or without setting GPS waypoints, with or without the audio bits that can be turned off. It happens on all cars. It's a systemic bug. Nothing the player can do.