The only issue is, for it to work, you have to give the site your login info for your bank accounts, credit cards, etc.
I was really apprehensive about this when I first heard about it, but a bunch of my friends use it, and there have been absolutely no reported security problems. Apparently, they use the same encryption that your bank uses, and access your information with a read-only connection, so it can see your account information, but can't do anything with it.
Apparently, it doesn't add any risk to that which is inherent to online banking.
