STL is a nice library. It allows you to quickly create useful data structures with very little coding. It saves you from writing a lot of code which is conceptually quite simple, but often a source of annoying bugs. It also handles memory allocation for you, so you can have variable sized lists and strings without the hassle of doing a lot of allocation and deallocation of memory.
In game development, many companies make use of STL in tools. It’s reliable and saves development time. Also in tools, we are often less concerned about memory and performance issues. In console game code however, STL is used infrequently.
I’ve met several interview candidates who I would consider overly reliant on STL. When asked to solve a problem which called for an array of non-fixed size, the candidate would not be able to come up with an alternative for an STL vector. Or to give another example, would solve a problem which called for a simple array by using an un-necessarily complex STL type, like a hash.
Being familiar with STL is a good thing. It’s a useful tool when used in the right context. Having the flexibility to use it or cope without it is the best position to be in. I think that a good check for yourself is making a rule that you’re only allowed to use an STL data type once you’ve implemented the structure on your own at least once.