While I haven't purchased in quite the bulk as some, I have used several of the surplus powder over the past 10 or so years. So far I haven't had anything go bad or produce anything out of the ordinary when loaded and stored for a while.
My uses have been mainly with the WC-860, and WC-870 for magnum calibers, but I have also run through some of the Salut, and a cople of jugs of the WC-297.
When I was looking to replace my stock of 296 I compared the prices and couldn't pass up trying the 297 for half the price of standard 296. It has been wonderfully accurate in my magnum revolvers, and with the lot I have is only slightly slower per caliber and weight of charge compared to my last lot of 296.
I do agree with what has been said about storage however, but I also have had newer off the shelf canister powders go south in a relatively short period of time as well, stored in the same areas as what my surplus has been stored in.
Like has been mentioned the prices have gone through the roof over the past few years, but if you find something you know your going to burn a lot of in a particular caliber, and the comparison saves you extra cash over the standard product, then why not give it a try. To me 5-10 dollars isn't going to do it, but 1/4 or more of the price on an 8# lot after shipping will at least get my attention.