I have a 7'6" Joy and I had a 6'4" Joy and both are (were) great. The 7'6" Joy is a super fun mid-length that has an amazing amount of versatility in terms of how you can surf it. The pintail gives the Joy a lot of drive and hold on the face that is really interesting to play with, and you can actually push it harder on waves than you would think. My 7'6" Joy is the perfect mid-length. Reef points seem to work best (Swamis down here in San Diego, for example), but I've had it out in beach breaks and it excels in steeper waves too with the right timing.
Someone else noted that the smaller length Joys had a different feel and I think that is entirely true. My 6'4" was amazing, but I noticed that I surfed it a bit differently than the 7'6". I had some great rides on the shorter Joy, but it actually seemed to fit somewhere between a mid-length and a shortboard. I ended up surfing it more like my shortboards, taking off closer to the peak/critical section. When I did that, it worked really well and was a blast to play with. With this approach I really reaped the benefits of the pintail design; it excelled with a more critical take off, because it gave me a lot of "hold" for a single fin. I ultimately switched out my smaller Joy for a Pleasant Pheasant but only to diversify my quiver.
If you are looking for a really versatile mid-length that can be both cruisey and a little more aggressive when needed I'd definitely recommend the 7'6" Joy. If you are looking for a really unique single fin surfing experience on a board that can be pushed in faster waves but still can get you down the line then try your hand at a shorter (maybe sub-6'8" model) Joy.
Either way, you'll be happy and racing down the line!