Never or ever? I have to use it for a work project. It's ok when dealing with numbers or matrices, but very painful in many respects. It's missing quite a few features found in more recent languages. I don't recommend it personally unless you're working with other big fortran codes.

* Missing standard data structures, like hash maps. I just wish I had something like a Python dictionary built in.

* Strings are pretty awful. You can now have resizeable strings but using them is very painful (and buggy in gfortran). You'll have to constantly strip spaces off the ends of strings to even print them.

* Some of the compilers are very immature (gfortran), if using any fairly recent features (e.g. resizeable strings). I've hit several (still unfixed) compiler bugs and my code contains some terrible workarounds.

* Available libraries are hit and miss, particularly outside the numerical area.

* Linking to C/C++ is possible, but painful with the binding support.

* Documentation (including books) just isn't good compared to what's available for C/C++/Python, etc.

* No templates (would come in very handy for some routines which handle different numerical types).

* Ok for numerics, though can't use C/C++ intrinsics for AVX/SSE maths.

I suppose what I like better than C/C++ is the module system and built-in support for N dimensional arrays with proper slicing, etc. The allocatable array thing is quite nice - it's like smart pointers in C++.