I don't have a problem with them if they are used responsibly and with consideration for other members of the public, but more often than not they aren't. I've just been bashed in the foot by a hulking great one in Morrisons, why the man driving it felt the need to use one of that size in a supermarket when they provide their own smaller and more suitable for going around shop mobility scooters, I don't know? I couldn't have very easily end up with a broken foot, and I should imagine an elderly person could have been quite badly hurt by it.
Furthermore if he'd actually looked where he was going or said excuse me, before just barging through I'd have moved out of the way.
Why are these things allowed on roads, pavements and in shops without any kind of training or licensing? Half of the people who use them don't seem to know how to control them properly!