I've heard only one tech expert talk about mint.com. Leo Laporte and that was about a couple of years ago.
He gave it a positive review back then but then again like I said that was a couple of year's ago.
Here is a 2007 review on the site's security practices (reasonably reliable source lifehacker.com): http://lifehacker.com/312083/is-mint-ready-for-your-money
Here is a 2010 article from a security expert (exhacker?) but I can't vouch for his expertise though the article seems credible in its own right:
Brute forcing the password is one way, but a bit ham-fisted. There is no account lockout or notification of failed attempts.
AKA, mint.com doesn't punish/block hackers trying an unlimited combination of passwords if they are trying to attempt an illicit invasion of your account.
STRONG PASSWORDS (weird combinations of numbers and letters or esoteric foreign words and number combinations are a must!!)
Be wary of logging on in public spaces with public Wifi. Especially if the transmissions aren't encrypted.
How to Protect Your Mint.com Account from Brute Force Attacks
Don’t use your regular email address, setup one specifically for Mint.com (you could have it forward to your real address so you still get notifications)
Make your new email address random so it’s difficult to guess.
Use a strong password
Change your password [on a regular basis]
Finally: Another 2010 article from a source I never heard of before...http://moneyning.com/money-management/how-safe-is-mint-com-the-security-questions-worth-considering/
Computer and Physical Security
To protect information, Mint uses 128-bit SSL encryption. This cryptographic protocols is considered standard for online banking and other transactions that include sensitive information. It is designed to prevent eavesdropping and tampering. Seeing that a tool uses 128-bit SSL encryption is typically a good indicator that the web application in question takes your information’s security seriously. Mint also routinely tests the various parts of its website in order to ensure that everything continues to work properly.
Mint also has physical security measures in place, preventing unauthorized access to the servers where your information is saved. That additional step means that even disgruntled employees will have a hard time misusing information.
Is It Enough?
Mint.com’s security measures are on par with those many banks take. It’s a system that does secure your information very well.