We haven't had any Yellow
Scooter reviews on the board so I don't have any opinion on them. The only thing I want to point out is that they are not a "preferred provider" which means you must be present for delivery and pick-up of the scooter (preferred providers can leave/pick-up from the concierge). If that's an issue for you, you're going to be better off with Apple, which is a preferred provider. If you don't mind actually meeting the delivery person/pick-up person, then it doesn't matter.
Whether a company is a preferred provider or not doesn't say anything about the quality of service. The preferred providers have a contract with WDW that gives them the ability to drop-off/pick-up from concierge, come into the parks if a customer's scooter breaks down in the middle of touring, and some other things that I'm not remembering at this point.