Here's my experience with them when I used them in March:
My biggest complaint with FP+ is the lack of spontaneity. My family is not a "super-planning" family. We like to go and have a "wing it" attitude; it makes the vacation more enjoyable. We decided to go a week in advance of our arrival, but we had our hotel reservations for a few month. We got our MagicBands at Port Orleans Riverside upon our arrival, so we planned our FP+s the night before. Some attractions we were able to get decent times for, and other were either not available or terrible times. The new FP+ system does not allow for people like my family to continue the way they like to do Disney. My next complaint is planning FP+s between multiple groups. Halfway through our trip we had relatives join us. Planning FP+s between our group of 3 and their group of 4 was a huge pain and stressful. There is supposed to be a way to link the groups together in order to make FP+ selections together, but it did not work for us, so we had to do it over the phone with each other. We were not able to get the same FP+ times for each FP+, and it was just a big pain. Another complaint would be the FP+ lines. They are constantly backed up because of how much slower it is to get through the beginning of the line. While it's not a problem once you get through the scanners, it creates a problem in the middle of the pathways because of everyone backed up trying to get in line.
I had one other complaint with FP+, but I forgot it as I was typing.
Overall it was not a terrible experience, but I would not say it was a good one, either. I would take the old system of a plastic card and FP over the new system any day.