In general, almost useless for online transactions (the whole point of this sub-thread).
Apart from that very low worldwide acceptance, and even when it's accepted in theory, it rarely works in practice in countries not accustomed to them. For example when I go to the US I often see the Maestro logo on various retail shops or PoS machine, but I have never been able to successfully make a transaction, ever. In fact, even ATMs are sometimes hit and miss, though those usually work.
Oh yeah, and they are in general unusable with hotels and car rentals, even in Europe.
It's the same with Amex in Europe. Almost all big supermarkets (Spar, Merkur, etc) display Amex sign on their entrance, but I have never met an employee that knew what to do with Amex cards. At least Amex cards, unlike Maestro cards, have, or can have good benefits like airline miles and whatever. You get nothing with Maestro.