I use a wheelchair when I'm flying anywhere - mainly because I can't walk the long distances to get to the aircraft and you don't have to wait in enormous queues for passport control and the like. When you have difficulty walking, you usually have difficulty standing for long periods of time and this service does help with that. I've always found the service to be great.
As Marg said, make sure you give them plenty of notice. I would also advise that you get cabin staff to confirm before they leave the aircraft, that there is a wheelchair waiting if your flight is not direct. I once flew SAS to Finland, changing at Copenhagen. The car to take me to the disabled waiting lounge didn't turn up (they'd forgotten about me). Myself and my friend were left like lemons on the tarmac next to the plane and the cabin crew bugard off and left us to it. The plane was refuelling and we got scared. We were there for half an hour. Eventually a maintenance man happened to drive by and took pity on us drove us to the disabled meeting point and we just about made our connecting flight.
I have to say that has only happened once, and I've never experienced any problems when flying to Germany, Ireland, Austria, France with direct flights, but just keep in mind if your not flying direct, to get the wheelchair/car confirmed before anyone is allowed to leave