I'm looking to get a DJI Phantom. I have a Walkera QR X350 Pro, but I find it a little quirky to operate.
Since I already own a GoPro 3+ Black and a gimbal mount, I was thinking of saving some dough and buying the Phantom 2.
Is this a mistake? Are there known problems with the 2 that were fixed in the 3?

The Phantom 2 is pretty close to end of life, a Phantom 3 has so much more technology than a 2.
With the 2 you will still need to buy a FPV system (RX/RX monitor) and you still will not have any camera controls.
You can download the DJI GO app and see how incredible it is, I think if you spent some time in the app you would really appreciate it.

Absolutely Phantom 3 series.