If one of the good combo cards is in the kingdom, Capital is going to be important. Now, assuming those cards aren't in the kingdom...
Capital is really good in Tournament games and Colony games. It could be essential on Dominate boards if you'd have trouble hitting $14 otherwise. Also it's crazy with overpay, for a big Masterpiece or possibly Doctor. Basically, whenever spikiness in your money distribution is what you want, Capital is good. If you're paying off at least 3 of the debt every time you play Capital, it's not doing much for you (with a few exceptions, like Storyteller maybe). In that case it's more like a Contraband with a different kind of penalty (-$3 in the future and/or now). You really want to go all in on the debt to get the most out of it. The price points of the key cards on the board will determine whether Capital is great or mediocre.
Also, any time the debt is not much of a drawback, Capital is helped. If on your next turn, you hit $7 and have 2 debt, and were going to buy a $5 anyway, the debt didn't hurt you. Whether you can count on that situation happening is another question. Or if next turn you're going to trash and pass with $2, you can just pay off debt. Works great with Mission, if you have at least some other reason for doing a Mission, since those coins were going to go to waste otherwise. In any case, the strength of Capital is roughly the same as the strength of the concept of "Debt" in the particular kingdom. I think it's underrated somewhat, because Debt is pretty strong.