Here's how I wound up using G+ and IA$; I only used G+ on my last day, and that is because it was an AK day with park hopping later on. I definitely see it as a FP+ downgrade, and it made me miss FP+ even more than when there was no FP or LL at all. I'd rather just utilize virtual queues and wait in line. Having said that, that IA$ is worth it if you can afford it.
I used IA$ for both Rat and Rise. We were getting in early enough on our first day to go to EPCOT, so I grabbed a time for Rat for the evening early in the day so we could ride that. It lets you straight into the interior area, so it is a really quick wait with IA$, a lot quicker than the virtual queue.
The next morning was DHS, so bought an 10:30 am Rise IA$ (and, yes, with IA$ you can pick your time, but it will move you later before you confirm if times are filling up). I can also confirm that it can change your time during the day, I think ours was moved maybe by 10 min or so later. We rope dropped and rode Runaway, and did anything else we wanted. Our time came for Rise, and it was down. The time for our return window passed, and the IA$ was replaced with an any time LL for Rise that day. In short, if the ride breaks and is down through your window, you can come back at any point in the day and go through the LL. I looked at the average down time for Rise, and it was listed as about an hour, so we just waited and got right in when it came back up.
The last day was AK in the morn and went ahead and got G+ for the day, and started off booking for our second park; MK. Rope dropped AK, and was in the first ride of FoP, off of that an on to Navi. Back to hotel, then toured all the monorail hotels, and booked my G+ selections at 2 hour intervals for MK, then lunch, then on to MK. Did what we wanted there (left before the fire excitement), then on to EPCOT where I had a G+ selection for Soarin, and the Land.
The lack of ability to pick a time for G+ severely hampers any type of actual "flow" for the rides you want (as does not being able to hop before 2pm). FP+ was smooth, G+ is jerky lol. To make the most of it, you have to track return times, and coordinate your picks that way. Of course the popular rides move a lot faster through booking windows, with the least popular going at a snail's pace. The only "advantage" that I really saw for G+ over FP+ was the ability to overlap ride windows.