The Magneride struts on my 2015 Sierra Denali leaked all the fluid out at 40k miles. Dealership refused to work with me at all and wanted $3500 to replace just the front struts. I'm getting them replaced at Firestone for around $800 with a lifetime warranty. Not worth the fight with the dealer. I'll let you know how the aftermarket product works.
10/25/18 update: So, I wasn't informed that Firestone was going to put Rancho replacement struts on and bypass the magneride system. I declined that option and had them put on new OEM struts fo r$1700. Rides great now, but ouch!