I've never dealt with any personally other than the filly I bought like 5 days ago.
But I can also tell you that they are very easy keepers and are susceptible to the easy-keeper conditions like founder and Cushing's. I have also been told a couple of times that Fjords tend to lack the pony attitude that Haflingers commonly have.
Eep. I am excited for mine to get big. She's maybe a yearling and hasn't been handled much. Catching her takes a bit of effort, but once she's caught she stands without issue. We're working on leading, where stubbornness has already shown, but she is slowly understanding.