Here is my recent experience... we are going March 5-18. No parks on arrival day, so I booked the throwaway night for March 6. I just finished booking all of my FP+. For my family of 5, the total cost with tax was about $103. Parking ($40), Magic Bands (5x$12.99=65) and I've more than broken even. I was planning to buy Magic Bands this trip anyway, so it was a no-brainer, and my kids are super-excited to have them. Early FP+ booking was just a bonus. We have 10-day tickets, so it was a big help in my mind.
I was able to get:
7DMT x 4 (one for each day at MK)
FEA and everything else at pretty much the exact times I wanted
I even got FoP at exactly 60 days for the 6th park day of my trip, Sunday March 11 - the start of spring break season (6:35PM for a party of 5)
In the past, I have had good luck with getting most of the FP+ I wanted at 30 days, and same day FP+ in the parks. Recently, it does seem that FEA is relatively easy to get at 30 days, while 7DMT has been harder (even at 60 days, there were days when the earliest times available were after 1:30PM), and FoP is virtually impossible. Although I do have friends that snagged same-day FP+ for FoP when they were there in October, so it does happen.
When I first tried to book the campsite online, I had the same problem as a previous poster. It wouldn't go through, no matter how many times I tried. I logged in under my husband's account instead, and it went through. Not sure why that would make a difference. The Disney website has been wonky for me lately anyway - I've been getting a lot of "ate the page" messages, don't know why.
Anyway, the throwaway night is definitely worth it for us, as long as I'm able to do online check-in and our magic bands show up when they're supposed to. I'll report back if anything goes awry.