This kit is quite different then your standard GW kits. Pretty much all the joints are posable, and all the pistons move. Because of this, each assembly section takes quite a bit longer then you'd expect. One of the arm joints has nearly a dozen pieces, all which must be fitted just right. Luckily they use screws for the tougher sections, which not only let you set the tension of the joints, but prevent you from making a model destroying super glue mistake.
I would not recommend this kit for a beginning modeler. You'll tear your hair out. It definitely takes patience, but you'll be rewarded.
Here is he next to an ironclad, megadread, and deff dread.
Luckily there are multiple planned replacement arms (of all manner and kind) that will be available this summer. I'm hoping for a more gauntlet oriented one, so I can have him hold a proper sword. Keep in mind the above link is only for the 28mm.
The 15mm arms aren't available separately yet.