How painful is it to use multiple Apple IDs with the MAS? I have two different Apple IDs, associated with Apple Stores in different countries. On my iPhone, some apps were purchased with one ID, some with the other. This used to be really painful in iOS 5, but it's not so bad in iOS 6: the system detects all apps that need updates, no matter what account I am currently using for the App Store, and it asks for login information for the other account when it needs to update an app purchased with it.

On my Mac, all apps were purchased with a single account (including Mountain Lion). However, the other account is associated with the US store, which tends to have better prices, so I'd like to buy some apps there. But how bad is the experience going to be? Is the Mac App Store going to detect updates for apps in both accounts, and install the updates with minimal fuss? Or am I going to miss out on half the updates, potentially including OS updates?

I would like to understand the exact experience I'm facing before I go this way, because I don't want to have the same mess I had with iOS 5 on my Mac.
Not really, I personally used multiple account. You see update notifications for both accounts; however, to perform the updates for apps purchased with another account you have to manually log out and log into the other account.