One thing about flying standby on a friend's pass is that making connections kind of defeats the purpose in many cases. While you'll have more options for flights that way, it's also doubling your cost(to the point at which the allure of cost savings is often negated). You also run the risk of being stuck somewhere other than your home or actual destination.

That said, when it comes to a place like Vegas, flight loads will depend heavily on what day of the week you're trying to fly. If you're expecting to leave Friday morning/afternoon and come back Sunday or Monday...good chance you'll run into problems. During the week you'd probably be alright.