I came to the marble palace on my last day in Bangkok. After learning that my ticket to the Grand Palace included entrance to this attraction, I figured I had to do it. First and foremost, the throne hall is open during a shorter window of time than the Grand Palace, so plan accordingly. It's also a great place to hide from the sun and it's fully air conditioned. I also spent at least two hours here.
There is a dress code, similar to the Grand Palace; full length pants and a top that covers your shoulders. You will also be asked to place your belongings (including cameras and phones) in cubbies they provide on-site at no additional charge. Security will ask you repeatedly if you've stashed away your phone; I highly recommend complying. They will also provide you with equipment for an audio tour.
The throne hall is really a beautiful piece of architecture that serves as a museum housing some of the greatest treasures Thailand has to offer. The collections include thrones, crafted ship models, fine tapestry murals (incredibly large), enormous and intricate wood carvings, royal howdahs, and other personal goods of Thailand royalty. All of the objects are handmade by groups of artisans, who spent years crafting each exhibit as a gift to the royal family. There are dozens of pieces and each one has a history and a story (though the audio tour can get pretty repetitive - same discussion format and monotonous). The last exhibit is an art gallery (tapestry) of award-winning works from Thai artists.
While I'm not an art connoisseur, I think anyone who enjoys the highest level of craftsmanship will find this attraction fascinating. I rate this attraction five stars as a testament to the incredible craftsmanship every work embodied.