When should you use an STL other than the one that comes with your compiler?

I was curious about STL implementations outside of what's packaged with gcc or Visual Studio, so a quick Google search turned up a few results, such as:

• Apache stdcxx
• uSTL
• rdeSTL

Under what circumstances should one use an alternative standard template library?

For instance, Apache's page has a list including items such as "full conformance to the C++ standard" and "optimized for fast compiles and extremely small executable file sizes". If it's so good, why wouldn't it replace libstdc++?

• STLPort
• STXXL (which is sort of special purpose, meant for large data sets that won't fit in memory)
• Dinkumware (commercial)
• SGI STL
• libstdc++ (GCC's implementation)