How do you win?
1) Did you have fun?
Players win if you're welcome at the next session.
DMs win if there IS a next session.
Fun has been a measure of success in RPGs for me for a long time, and I have never forgotten what happened many years ago (school days) when our DM decided to put some variation into our D&D campaign by taking a break and letting someone else take the hot seat. I went from good player to bad DM and 3 weeks later my badly-plotted adventure had been dumped in favour of Cyberpunk. What makes it particualrly painful was that we were most of the way through the Master level modules and on course for the big finale - whatever that was going to be.
2) Do you care about your characters, or how much cool kit they've got?
The other big factor (for me) is how involved you are. I used to play in an excellent Mage:The Ascension campaign that flowed entirely out of the head of a highly creative GM. He didn't just develop well-characterised opponents and devious plotlines but pushed & helped us to flesh out our character's life stories. He would then use that information to enhance the game world. To be suddenly confronted by the ghosts of your character's past is an exhilarating feeling.
When the game suddlenly beomes about WHO your character is (and what is ROLE playing about if not character?) and not how much damage he can do with his magic sword or a wave of his hand then I feel you've achieved something.
Albedo
(Bit long for a first post, but I get carried away sometimes)