I'm home from my trip and although the park was packed, we used SDFP all week and it worked fantastically. Just a few observations I (and others in my DIS group who were traveling at the same time) made:
1) if at first you don't succeed, refresh. seriously people are dropping FP's all the time and availability seems to change second by second. this applies both for 4th FPs on your primary account as well as SDFPs at the kiosks (it is obviously easier to do from your phone). I picked up a lot of PRIMO FPs by refreshing a few times and grabbing it as it popped up.
2) SDFP availability is fantastic in the mornings. So good in fact I was half wondering if they don't hold some available for people to book day of. On my Epcot day I planned to RD test track then go to soarin' but TT was down when I got there and instead I went to the FP+ Kiosk and I managed to score both TT and Soarin' on two different sets of media. the awesome tier 1's early in the day were gone a few minutes after RD though.
3) if you are traveling with kids have an easily identifiable way to tell them what to scan. We had "regular" bands, "christmas" bands and cards that I kept for the third set. Only once did my kiddo scan the wrong band at peter pan and the CM freaked out a bit "WHO IS J???" (the name on the band) because it was super crowded and it was really backed up and people were pouring in. I just said "he is wearing a friend's band and scanned the wrong one, he is <name>" and we moved on. Poor kiddo got a little freaked though, lol.
4) I will say that this trip was somehow more exhausting than others. We did TONS of stuff. We did soarin x5, we did TT x 3, we did all of the mountains x2 and 7DMT x3 and TSMM x3 etc. I found that we had to work harder to fit in the minor stuff that we so enjoy because I kept scoring FPs for the 'good' stuff. So just something to keep in mind about your flow, you may need to dial it back a bit if you want to experience the parks in a more laid back way because it is so tempting to rush from primo FP to primo FP.
5) my kiddo is on the spectrum and the lack of waiting really really enhanced our trip a ton. we didn't end up having to use
DAS as this worked much better for a kid who has a hard time waiting.
6) i liked having only one set of extra MBs and another set of cards (his grandparents' used up ticket cards), it made it very easy to keep track of what was where. the extra mbs showed up on my phone and were linked to my MDE, cards were not.
7) it was not bad at all keeping track of the various FPs in this way