You can actually set the FP+'s to the times you want and they don't have to be within a certain time block. ; While the reservation system does try to line them up, after you initially lock them in you can go back and edit them one at a time, moving them to whatever time you wish that's still available. ; However, the interface to do that is clunky and takes way too many clicks to accomplish, especially if you are trying to flip the times of two FP+'s. ; If you're trying to do that, you have to move "A" into a completely random spot, then move "B" into the spot originally occupied by "A" and finally go back and move "A" into the original spot occupied by "B". ; Way too much work for what should be a simple operation.
My biggest complaint is that you are stuck with 3 from the same park. ; Like Gary, about the only thing I would use it on at DHS is TSMM, and wish I could use the other two in another park on the same day. ; At a minimum, don't force me to take up spots at other attractions that I don't have any intention of using.
I will say that one thing I like about the system is the notification they send out when an attraction you have a FP+ for goes down. ; I had that happen in September and the email they send out states that you can use your FP+ on the attraction anytime later in the day if the attraction is running again, or you can use it on any of a list of attractions. ; If I remember correctly, the list at DHS is pretty much all of the attractions currently equipped with FP+