Much is in the FAQ (see sticky).

In a nutshell: there are no complementary upgrades TATL. If you waitlist, elites will be ahead of you. If you're willing to shell out $$ for full "Y", note that depending on the dates of travel a discounted BF fare might actually cost less. As to a throwaway return, that depends. Lots of threads on that as well. Pros/cons really come down to the specific fare rules, and whether you want Onepass miles or not.

One more point - you're not going to be pleased if you pay full Y and don't clear. Might as well go for the lowest fare... othewise you're really gambling and may end up spending a whole lot of money on a more expensive coach ticket.