One of my players was generating a new PC, one with an Obsession. This prompted me to take a deeper look at the disadvantage and decide how I expect it to be played.
Obsession is greater than desire. This isn't a quirk. It's not just something you'd like to do, if and when it becomes possible. This is something that consumes you. You need to make a self-control roll if you have a chance to get even a tiny bit closer to your goal - and that's even if you are wrong about this thing getting you closer.
Points Are About Goals - The points you get back for your Obsession aren't determined by the level of obsession, but rather the attainability of what you are obsessed with.
Want to slay a dragon? [-5]. It's tough, and you'll make dangerous decisions and might risk a fight you shouldn't to get it done. Want to find the spells of the ancients? [-5]. I mean, who wouldn't want to find the spells of the ancients? You're just going to give "we might find those spells" the call over "or we can just go get that treasure over there."
Want to become the most powerful wizard in the world? [-10]. That'll take a while, there is a lot of competition, and lot of dangerous shortcuts that will tempt you along the way. Even then, you're more likely to die than to succeed. Want to destroy the cult of the cone-hatted folks? [-10]. Even knowing how many there are is a challenge, nevermind exterminating them them root and branch, down to the seeds of their regeneration. You'll equally make some bad decisions when you get a chance to do something to harm them.
Self-Control Rolls Aren't For Always. You are always obsessed. But if you always choose to make your roll, and don't pursue the obsession sometimes without rolling, you're really just better on rolling well for some free points. I'm not a fan. Give in without rolling whenever you feel like it, or see a good chance to do so. Sometimes your paper man just wouldn't try to give in.
Conflicting Disadvantages Are a Thing. If you have disads that conflict with your Obsession, remember how that works. Only Sense of Duty gets to stomp on Obsession freely.