My wife uses a wheelchair due to not being able to walk long distances, so far travelling mostly with EVA Air and also Thai Airways has been very positive. I always add her wheelchair use to the booking and have yet to be disapointed. On check in just let them know you'll be dropping the wheelchai at the gate so they can tag it appropriately.
At LHR it's a bit of a shambles if you want to wait for a buggy ride, so I normally push her to the gate which helps keep me fit! Arrival is sometimes a little bit delayed while they retrieve the chair and at BKK there's always an airport staffer to do the pushing (and yes we tip him) LHR is self help but they do direct us to the buggy immigration gate.
Last time departing BKK they were a bit short of staff so rather than waiting for staff I just pushed her myself, same as at LHR, arriving in the UK is the same as arriving in Thailand.
Mostly one of the cabin crew will ask us to stay seated onboard until they've retrieved the wheelchair and it's ready, if people are stil deplaning they'll sometimes pause them for a minute so we can get to the chair or wait until evereyone is off.
Although ours does fold it's still quite awkward, never even considered the possibilty of taking it onboard, my wife is OK walking on board to her seat, airlines do have special narrow wheelchairs available for people who need assistance all the way to their seat.
Andrew