I was being conservative on the 15 minute time frame, the common estimate is about 10 minutes. Keep in mind that this is the amount of time to try EVERY possible combination, including all the ones with 2 or more simultaneous numbers. What are the chances someone will need to go through all of them before they happen across the right one? It's very easy to go through a lot of combinations very quickly, especially if you have a list in front of you.

http://www.tech-faq.com/how-to-open-a-simplex-lock.html

This doesn't mean they are bad locks, you just have to know the inherent limitations of your choice. Simplex locks are great for access control, not so much for security. You'll notice you will only see them used on small lock boxes, there's a reason manufacturers don't use them on larger safes.

As I stated, it would be fine if you needed to keep small children out of your safe, but I personally would not trust one where a bored/curious teenager had access to it, because they will get into it. It boils down to who you're trying to keep out. You don't want to give yourself a false sense of security, that's almost as dangerous as having none.

I prefer mechanical dial locks for their durability, but for frequent use an electronic lock is a lot more convenient.