I agree that the FP is worth it, but ONLY IF you're dead set on meeting him in MK. We had a FP to see him on our trip in early May. When our time came around, it had been absolutely downpouring for close to an hour, and people were leaving the park in droves. But the line for Mickey was still 45+ minutes standby.
But if you don't care where you meet Mickey, there are plenty of other options to help you save the FP for a ride. My kids met him at Chef Mickeys and Garden Grill meals, and randomly in AK about 20 minutes after park open when we were walking by and the CM told us there was literally no wait. The only benefit to the MK meet right now is that he and Minnie are in their birthday outfits. Personally, I think their safari photos with Mickey and Minnie at AK turned out cuter, and the interaction at both character meals was better and more fun than the MK Mickey.