The downside is that the O-rings wear out pretty fast compared to the cup piston.
This obviously means you have to replace the O-ring often. The best thing for that is to use a pivot pin with E-clips or some fastener to facilitate rapid removal.
The stock Crosman roll pin should be ditched, a solid pin in it's place.
Charles Mellon offers a great piece for this purpose.
Or you can make your own, like on this one :