Among other issues, I have RA, bone spurs in my heels, and Plantar fasciitis. Today the bone spurs and PF are behaving themselves and not causing any issues, however, lots of walking, such as that found at WDW will trigger them -- the first bout of PF I had was triggered by and started at WDW (nearly ruined that trip). My RA, which is across many joints, also does not like lots of walking, nor standing, nor... you get the idea.
In order for me to "do" WDW, I need to use an ECV. I do not use one in normal life [except when visiting large zoos for many hours] nor do I use other mobility aids in normal life.
When I fly, the stiffness and pain that sets in from being seated on the plane means I need wheelchair assistance for connections and for the destination airport. I arrange that through my airline. In my early years with RA I didn't need it, but the last several it has been a necessity, unfortunately. And the couple times I have stubbornly tried to walk the connection myself it has not gone well even if I did make it to the gate. I've learn my lesson and don't try pushing it. Note that wheelchair assistance often takes MUCH longer to get from gate to gate than normal, so I ensure I have long connections, at least 2 and preferrably pushing 3 hours long, especially when clearing Customs is involved. FWIW, having a NEXUS card helps with this as it makes us eligible for priority security lines [in the US, for TSA PreCheck] which are often (but not always) faster than the normal lines as well as eligible for Nexus (Canada Customs) and Global Entry (US CBP) lines for clearing customs, which are IME always much shorter. Sometimes the wheelchair assistance messes up, so I am also prepared to walk it if needed, which means comfy clothes and shoes and an easy wheeling (4 wheel spinner) carryon and a personal item that can attach to the carryon so I have fewer things to carry. I have discovered the extended handle of the spinner carryon can act as a sort of mobility aid in a pinch.
As for WDW. The distances involved at WDW are tremendous. For example, just walking around World Showcase, WITHOUT going into any of the country pavillions, is OVER ONE MILE (nearly two km!!). Typical distances walked in a WDW day can easily be 6 to 8 miles or more. And all that walking is mixed in with a lot of standing relatively still.
Our recent trips have been a WDW resort stay, then a
DCL cruise, then a WDW resort stay, with us visiting parks during only one of the resort stays, with both stays being a lot about spending time just relaxing and enjoying the resorts. Our last trip we had 3 nights AKL, then 10 nights on DCL, then 3 nights CCV. The only park we visited was one
MVMCP during the CCV stay. So for that trip, I only got the
scooter for our CCV stay, as I could manage the AKL stay without one because we had so much down time and could manage our days in limited chunks that were doable for me. YMMV.
So for the park visit portions of our trip, I rent an ECV from an outside vendor. My current vendor of choice is Gold Mobility Scooters. I have previously rented from Walker and Buena Vista. See my previous post about the "tiller issue". Renting form an outside vendor means I have the scooter at the resort, for the buses, as well as for the parks. If all things are equal you have two basic choices for an outside vendor: (1) Scooterbug, who is the only preferred vendor with Disney, which allows them to leave and pick up the ECV with Bell Services even if you are not there; and (2) another vendor (of which there are many choices) who must arrange to meet with you in person to pick up and drop off the scooter. There are a host of pros and cons of various vendors and such, which other threads discuss.
As for using the ECV at Disney. I have found it greatly increases my independence and makes the trip go much better. I am in a LOT less pain than not using it. I can go most of the day at a park, and it is fatigue that eventually gets me, not pain. WITHOUT the scooter, by two hours I am in severe pain, three can barely walk, and at four have to make numerous stops and sit then try walking again (in severe pain). I made the mistake once of "its only a few hours in the park" and left the ECV in the room and will never do that again, as that is when I learned just how long I can go... :-(
As others have noted, the ECV can go through almost all the ride lines. I will often ride the ECV to an "area" (e.g. Tomorrowland near the Peoplemover entrance) and "park" it (near peoplemover there is a parking area just for wheelchairs and scooters, which works great -- in other areas sometimes it means parking in stroller parking). I then walk to one or more rides depending on how I am feeling. Then I take the ECV and ride it to the next area or next ride, again depending on how that day is going. If I am having a really bad day, then the ECV is going into the ride lines with me. A really good day, and I can park by Peoplemover and do most of the rides between it and the bridge to the hub by walking.
So, I have a great deal of control over HOW I use the ECV. If I want to be walking, I walk. If I need to be riding it, I ride it. I can adjust on the fly as needed throughout the day. *I* am in control. And not dependent on someone pushing me and their energy levels etc. Also, my sister can go off and ride Space Mountain or the other roller coasters [which I can't because most rollercoasters are a good migraine trigger for me] and I can go off and do something else, or just sit and relax for a bit on my very own portable comfy ECV seat
As for pain management. I find using the ECV well [and not being stubborn about it -- USE IT; and stop walking so much and use the ECV more when pain starts increasing] prevents a lot of flaring and thus pain. My complication is I have frequent migraines and am severely restricted on how much of ANY type of pain medication I can take as a result [even Tylenol]. I am also allergic to NSAIDs, which leaves me with Tylenol as my option. But I spare taking it so I have "days" available for migraine relief meds, which basically *for me* means no pain medication for RA or PF. Ice and heating pads are my friend. But getting my disease as stable as possible in partnerships with my doctor, and then doing what I can to avoid flaring it are the biggest things I can do.
Hope this helps.
SW
It is true - the seat is not very deep nor are the wheels very large. Which makes it easier to get dumped. It worked for me because I am very active and even with the health issues I was dealing with, still very nimble. I paid close attention and was always ready to hop off when I saw big bumps coming. DH did dump me a couple of times but I was always ready and hopping off was simple. I agree with you that it is definitely a use with extreme caution thing - not good for anyone frail or too weak to move quickly when needed. 
