I have owned a 590A1 for four years now. I absolutely love it. I have the 20-inch model, and the capacity is great, I keep 7 rounds of 3-inch magnum 00 buck in it for home defense, but it will take 8 of the 2.75" shells. I believe the 18.5" 590A1 holds 6+1. The ghost ring sights on mine are phenomenal for shooting slugs, I can hit targets all the way out to 100 yards with slugs, it's almost like shooting a rifle. The slugs + ghost ring sights combo brings a lot more capability to the table.
I've abused it and run it hard and it has never malfunctioned. The only thing I did to mine was add a GG&G sling mount for the front end. And I am waiting for MagPul to release their Mossberg SGA stock, the OEM stock is too long for me.
The 590A1 is my one and only shotgun and will stay that way forever. I don't need any other.