I've been using Mint for a little while, and I like it enough that I want to convince my wife to jump on board.
From what I've seen, though, there's no good way to have keep track of accounts where the asset/liability is shared.
Let me be specific. We have a shared credit card and a shared savings account, but our paychecks come to our personal checking accounts. Monthly, we both pay half of the CC. There's no way to say "half of this debt is mine" except for splitting every single transaction. Therefore, there's no way to calculate our individual net worth and things like that.
Basically I guess the problem is with their data model, that one login represents one legal person. There's lots of feature requests for this sort of thing, but I don't expect it to happen any time soon.
Bonus feature: forecasts. I want to be able to define a bunch of recurring transactions and then ask what my various balances will be at some point in the future, assuming some interest rate.