I'm currently using Atmel SAM series. The choice was easy: IDE is Atmel Studio which is very good, free and I used it for developing AVR before. The debugger I have is the Atmel ICE which I also used for AVRs and it supports the ARM series as well.
What makes the STM32 series interesting is the very low cost for the chips and the development boards. However, what IDEs people use for developing? I am interested in something that is completely free as mainstream as possible so it will be easy to find help when needed and also to make sure the tools stay up to date. I could find a definitive answer on Google or on ST's website so if someone is already involved with developing on this platform I would appreciate if you share the data and your opinions.