I am new to the Arduino system.
But I am an experianced AVR programmer using the Atmel AVR IDE.
All my previous work has been done in assembly language.
But I have been writing Windows code in C++ for many years.
How difficult is it to use the Atmel IDE and download the code with the Arduino bootloader.
I do own an Atmel AVR programmer so I can do it that way if I need too.
It's a matter of taste whether you find the Arduino IDE great, or at least you can live with it. Find out yourself, examples for all libraries are included. And understand that "sketch" is only a buzzword for a program. And the created code is not optimal, there are many expensive tricks used with e.g. unified pin numbers.
If you prefer full control over compilation and build process, stay with the AVR Studio. Arduino and Studio use different libraries, you should decide which one to use before starting your first big project.
Just my 0.02€
If you feel more comfortable with assembly language and you want to get started with Arduino, note that with the Arduino IDE, you can mix C/C++, specific Arduino functions (e.g. digitalWrite(), analogRead(),…), inline assembly language and pure assembly language in the same sketch .
The great thing about the Arduino environment compared to every other "educational" system I have investigated is that you're not trapped in the walled garden. If you want to toggle a pin with a direct bitwise write to the port then you can do that without knocking down the other nice parts of the environment like the convenient millis() timer.
So if you're an experienced AVR programmer, you should fit right in once you discover the parts of Arduino that are actually useful to you. You don't have your hands tied. The hard part is learning to reach out and find what parts are useful. I've seen a few while(1) loops created by experienced programmers that are counter-productive to the Arduino way of doing things.