We just spent the day at MK and used FP+ as offsite guests. It worked perfectly. We rode Big Thunder Mountain, Peter Pan and Space Mountain using FP+. We had our tickets in those clear pouch type lanyards and found we didn't have to remove them from the pouch. They scanned just fine. I set the first FP+ ride for 11:05. We saw Cinderella, then we rode Pooh. Pooh broke down so the gave us these paper FPs we can use on certain attractions up until March 31. We went to Ariel's Grotto, rode Under the sea, rode its a small world, rode haunted mansion and the went to our first FP+ at big thunder mountain. Then we watched Mickey Philharmagic. We used our second FP+ for Peter Pan. By now it was freezing and everyone was getting miserable so we went in gift shops to try to get warm. We hung around and did Space Mountain and then came back for the hotel for a break. I think if we wanted to do more attractions it, it would have been fairly easy. We also didn't use the paper FPs. Maybe next time.
So far I think FP+ is great. We pretty much walked on to every ride. Ariel's grotto might have been the longest we waited and it was only about 5 minutes.