To answer your last question first, yes, they are heavy fuckers.
They're worth it though, especially when if you can find the coreupt or dynastar branded ones for like $120 online. They have the all metal pivot toe, which is super simple, bombproof, and just ridiculously solid feeling. The heelpiece is mostly metal, definitely not as solid feeling as the pivot heel because it's on adjustable rails, so there's always a bit of play. Px12's are lighter than the 15 with their plastic toe, and I'd say the px15 is pretty similar in weight to the pivot 18.
They were my first legit binding after skiing on explosive marker free's for a few years - metal bindings are really one of the best investments you can make.
Also, someone compiled a really awesome binding spec spreadsheet, I can't remember who though.