I used it before I understood anything about databases, and I got some jobs done with it. Now that I have some knowledge, I choose PostgreSQL for most cases. But mongodb got me up and rolling in a pretty DRY way when I first started programming for the web.
If you get to the point where your website is outgrowing it’s initial mongodb implementation, you have a very good problem on your hands.
It doesn’t take a lot of traffic to outgrow a poorly implemented mongodb structure with a lot of data. That’s not a fault of mongodb as much as it is using the tool wrong. When I first started using mongodb I treated it like a relational database and hit performance issues very quickly.
If you get to the point where your website is outgrowing it’s initial mongodb implementation, you have a very good problem on your hands.