I havnt used a 521 since last year, but recently I had to use one in a comp, all I can say is, the amount of tyre fold I got was ridiculous, it made me fall off in atleast 4 sections.
I also found my bmf rim stiffer than a 521 (maybe this was down to wheel build?).
I will NEVER go back to a 521, I think they are waaaaaaaay to narrow for natural.
If you ride street then I think they will be fine, but if you do comps or natural riding, go for a wider rim.
PS good luck in finding ready built wheels, I aint seen any for a while