I was there the 2 weeks ago, which was my first experience with FP+. I have to say it really didn't impede my spontaneity. Granted, I was one person, but that actually made me want to be more spontaneous because I didn't have to cater to others' wants.
The biggest way that I found it to be spontaneous is that I booked my FP+ for late afternoon, therefore choosing my 2nd park of the day in advance. But my 1st park each day I decided either the night before, or when I got up in the morning.
I also changed FP+ quite often (but not usually headliners) as I figured out what worked better than what I had booked. Again, with one person it's probably easier to do that.
I did NOT miss having to cross the park multiple times to get FP-, then to use FP-, then to get another FP-, and then cross the park again with no idea if the ride I wanted had FP- or for what time.
My biggest complaint is that I would have liked to overlap windows. If you have a FP+ for Splash at 5:00 and one for BTMR at 6:00 and you arrive at Splash at 5:00 you're going to have quite a wait for your BTMR timeslot to open. However, this is where I was spontaneous as well: I'd look at the standby time on Splash & BTMR and use the time I had to wait by doing one of those rides standby. If I'm going to wait around, why not wait in line?
And that's not really a complaint - more of an observation. I was worried about FP+, but I like it better than FP- and I'd call us super-users of that system.