Belshazzar was one of the first pieces I really got to know with the score (vocal score only I'm afraid), being an A-level set work when I was doing mine. I don't think it was the only option on the syllabus, actually, but seeing as I was born and brought up in Oldham and went to the school where Walton's brother had taught music, it must have been a fairly natural choice.
I think it's great. Maybe it is a bit theatrical, and certainly not groundbreaking, but it's pretty irresistible, especially since I can still remember large chunks of it from memory and sing along whenever I hear it.
My absolute favourite Walton piece, though, is the Violin Concerto, which is tremendous, and which I commend to all on, erm, r3ok.myforum365