i dont have a shotgun in 3.5 but i probably will when i get a new gun. I buy my waterfowl loads by the case and a case of 3.5" is only about $25 more then a case of 3" so thats not that bad. i would be willing to pay that.
I never was a fan of the 3.5" shell. IMO, it's nothing more than a 10 gauge load stuffed inside a 12 gauge shell. Sure, it has its purposes on the field, but it sure isn't something I'd want to be shooting all day long.
I find that the 3 1/2" chamber and loads imposed by the magnum shells require an upsizing of the receiver and gun as a whole to the point where the 3 1/2" gun isn't so useful anymore for any kind of walkin' about..... I suppose in a waterfowl blind or when sitting waiting on toms, weight doesn't matter so much and the extra payload/reach has its place.
I wouldn't call them faddish - just a solution for a small segment of the market that really needs an extra bit of reach (and likely more room for payload when using non-lead shot).
I don't think it's a fad. I like having the 3.5" chamber so you have a choice of ammo. I only use 3.5" shells for hunting turkey though, everything else (including HD) I use 2.75".