I haven't had any break [yet], but out of concern that the spokes are Shimano-proprietary and unlikely to be available at area LBSs should one fail while I'm out in the countryside on bike trips, I asked my LBS to order some stock parts (3 sets of 3 sizes of spokes, 2 sizes of nipples). It's been more than 6 months since I first asked--probably partly the sales staff not promptly ordering them, and supposedly Shimano is slow to make parts available for these wheels and sent the wrong color for one blade size for some reason--and I've only received 2 sizes of spokes and 1 size of nipples so far.
Tried a couple Shimano-authorized shops highly recommended in these forums, but they declined to help (perhaps due to small order, but the order will still be expensive).
Any of you have better responses?