They do have a nasty sting, but they're only aggressive near the nest. They're paper nest builders, so if you can locate the nest and avoid that area, you likely won't get nailed.
I got nailed twice walking out to the mailbox before I finally looked up. D'oh--the nest was in a branch overhanging the driveway...
My neighbor gave me this tip, and it works really well: For any
sting, remove the stinger if necessary, then cut an onion and put the juicy surface against the sting. Something in the onion juice neutralizes the venom. The earlier you can get the juice on the bite, the better, but even if you delay, it'll take away some of the pain.