I guess it depends on what you want to do. My most recent trip was decided on and booked within a few days of when I would be there. I didn't pre select any of my fastpasses, I did it the night before each day. I didn't go to a park on my first day so I picked fastpasses for the second day the night of the first day, picked for my third day the night of my second day and so on. I changed them around even before I got to the park the day of. I was able to do pretty much everything I wanted. I rode everything I wanted, most of them multiple times. The longest wait I had was 40 min at Rockin Roller Coaster. I was able to get a 10-11 am toy story fp the day of. The earliest I got to a park was probably 10 am but I did close them down. I only used my first 3 fastpasses, I didn't get any more after that.
I didn't do any table service meals though, which would probably be the hardest to get on a few days notice. I also went by myself so it may have been more difficult to get some of the fastpasses had I been trying to get them for a group. The only thing I wasn't able to do was get a reservation for BOG for lunch. It kept saying my reservation number wasn't correct and I didn't care enough to call someone to figure it out before I left. So that's more on me than anyone else. My family is planning on going sometime around Oct 2015 and I'm sure we'll plan to go then for probably lunch and dinner (or at least try to get in when our window opens to make reservations for dinner).
I felt like it was a successful trip with little to no prep.
I didn't do any table service meals though, which would probably be the hardest to get on a few days notice. I also went by myself so it may have been more difficult to get some of the fastpasses had I been trying to get them for a group. The only thing I wasn't able to do was get a reservation for BOG for lunch. It kept saying my reservation number wasn't correct and I didn't care enough to call someone to figure it out before I left. So that's more on me than anyone else. My family is planning on going sometime around Oct 2015 and I'm sure we'll plan to go then for probably lunch and dinner (or at least try to get in when our window opens to make reservations for dinner).
I felt like it was a successful trip with little to no prep.