Codevision is easy to use and produce good code. The best part it's the code generator for setting various parts of MCU (timers, initerrupts, ADC etc). Also have good support from the producer. Same low cost range is also ImageCraft with almost similar features. There is also GNU C compiler with the advantage to be free.
IAR is 10 times more expensive then Codevison. It's a little more difficult to acomodate with all the stuffy settings but it worth. Produce shorter code then other compilers (don't forget that AVR core have been designed together with the IAR guys).