I've had two-my first male is no longer with us but I have my female. My experience is limited but I still find somewhat similar to what others have noticed with them.
Sultans are usually quiet, sweet, and friendly birds. They're really quite nice, but mine have always been bullied in my mixed flock. Both for having a crest and for being generally submissive birds. Mine have never been hardy, having gotten sick a few times in their lives. Weather-related, they tend to do better in heat than cold in my opinion. But they're really okay in both.
As mentioned, mine have always been bullied by others. And they're so sweet, I hate seeing them victimized. They're supposed to lay roughly 70 eggs a year but my female is from a hatchery and tends to lay more. They are seasonal layers though.
They make excellent pets, and are, in my limited opinion, the most kid-friendly chickens of the crested type. My rooster was the friendliest rooster I've ever owned. He was so sweet, easy to handle, and never attacked me.
My female was the first bird I ever showed. And I don't regret choosing her for a second. She was also easy to train and manage, and was a favorite at small shows. I used her for showmanship as well and she did very well. Hatchery birds rarely meet standard, and my female absolutely doesn't-but she does happen to look better than the hatchery Sultans I've seen. My male had very poor type but I wasn't showing when I owned him, so it didn't matter. Here's pictures of both...
My male when he was very young.
My female when she was 4 days old. Mind the Polish on the left.
I think she was 9 weeks here...
About 3 months old, after her first bath
After we brought her home from her last fair. Not a big show, but she took Champion Large Fowl. It was a good day for both of us.
She's retired now but after a good moult she still looks the same.