I was extremely excited when I first saw this card. I thought I was going to immediately buy it in every Rogue deck I ever played. XP is incredibly powerful and fun in this game, after all. But the more I play with this card, the less I like it.
The odds that your character dies at some point in missions 2-7 of your campaign (if they die in mission 1 or mission 8, that's not really relevant for the Obol) are just way too high. Even if you run "I'm outta here!" and Cheat Death, I just think the risk is too great. Starting over midway through a campaign at 0 XP is a truly terrible feeling. In solo, maybe it doesn't feel quite so bad, but I really would not recommend this in multiplayer.
Note that "I'm outta here!" is far from a perfect failsafe here. Many scenarios don't have a resign space or have one that doesn't show up until the final Act. It won't save you then.
Moreover, Charon's Obol can also hurt you even if you don't die. This is because the Obol makes you play scared. Vastly increasing the consequences of dying makes it harder to play effectively. A very common situation in this game is the following: your investigator is severely hurt, down to 1 or 2 health or sanity remaining, but only a couple of turns away from a potential victory. What should you do? A normal investigator can go for the win. Sure, you might take a trauma, but that's not really a big deal at all. And you might win! With the Obol, you can't really do that any more. The consequences of defeat are too high to take otherwise profitable risks.
Furthermore, playing the Obol can force you to stuff your deck with durability cards (like the aforementioned "I'm outta here!" and Cheat Death), at the expense of better cards that actually help you win the scenario directly. This hurts your deck and can mean that your deck's power level isn't that much higher than a deck that doesn't run the Obol, but also doesn't have to run these cards.
Still, I can see the temptation here. If you like the thrill of risk and are OK with a strong chance of having to starting over, go ahead. But personally, I find the game much less fun when I'm running the Obol. And setting considerations of fun aside, if I were just trying to maximize my odds of winning the campaign, I do not think I would take it.