I don't think it is technically cheating (I mean I don't think it is a banned substance). I know of one club that brought doctor in to talk about not using it but his lecture was not the fire and brimstone one the coach was looking for. Most coaches I know really frown on it and if caught the swimmers are pulled from the meet. But I am pretty sure it is still used.
I am not a chemist. I am just telling you what I know. It is not a banned substance. Coaches don't like it at all but C4 in particular is used a lot before practices and workouts. Swimmers that I know see it as an OK supplement. I am talking about C4 specifically. The problem with inhalers is that due to some of the air quality in these pools, swimmers have trouble breathing. So I don't know who is using legit and who is using to get an edge.