I have to say, I have been impressed with it thus far. It hasn't been on my shotgun that long and has only seen a few hours of live fire training and a few more hours of dry fire training, but sometimes you just get good vibes about a piece of gear. The side saddle is made from aluminium and uses a length of rubber hose similar to surgical tubing sandwiched between the back plate and the shell carrier itself to retain the shells. The side saddle comes with a black rubber hose installed and an additional white hose that can be substituted in for a higher level of retention. I am currently running the white hose. It is really easy to change, just a matter of removing and replacing a few screws. The only downside I see to this system is that just like surgical tubing the "elastomer" as Mesa calls it will probably dry out after a while and have to be replaced.
Since it is such a long side saddle, I do have to orient the two shells nearest the stock with brass up or they will interfere will my activating the action lock lever at the back of the trigger guard. I am running Remington #1 buck which is a high brass shell, so maybe with a lower brass shell it wouldn't as big of a deal. Also, if I am shooting off my support shoulder I have trouble getting my left hand thumb up and around the side saddle to work the safety. Neither of these issues are a deal breaker, just things I am having to accommodate. Personally I think the 2 extra rounds on the gun are worth it.
I plan to run this shotgun pretty hard over the next 6 months to really get a feel for it. We will see how Mesa's Sureshell carrier holds up.