There's nothing wrong if you rent a mask or loan it from a friend. That is if it is the right size and fits your face. You also want a mask that is reasonably high quality and has no damages to it. If you dive a lot, then it makes sense to buy your mask. If you only occasionally dive, like a couple of times a year during vacation, then it might be more convenient to rent. However, do not expect these masks to fit as well as your mask would! The skirt over a period of time will begin to shape into the form of your face slightly. If you use somebody else's mask, you might have a harder time getting a tight seal. So, if you can afford one and if you will use it often enough, then it does make a lot of sense to purchase your mask.