How easy is it to transfer from programming Arduinos to standalone STM MCUs?

I currently find and using Arduinos very easy and have done alot of projects with them, but now I want to use STM Microcontrollers in PCB's ive designed.

How easy will it be for me to transfer from plugging in an arduino and typing code to using an STM surface mount microcontroller and telling it to control my circuit? Is the programming just as easy?

It's possible to add support for STM32 microcontrollers to the Arduino IDE, which allows you to use some standard Arduino example code and libraries (as long as it doesn't contain architecture specific code). This should provide a familiar API. There is a list of options here:
The two that look the most promising to me (I've never used any of them) are:

What kind of STM processor ? (They have quite a few!)
If you pick an STM32F ARM chip that has already had work done by the STM32duino, you'll probably be in good shape.
If you had in mind one of the wonderfully cheap STM8 chips, you would have a lot of work ahead of you. (To start with, I don't think it even has a readily available C++ compiler.)