I uses ISIS. It simulates the code right on the computer, complete with SPICE for the electrical schematic. With PBP, I've got to watch the compiler asm output, but I can see all of the variables change and i/o pins as fast as I want, with breakpoints. When I use Proton+, I get to step through the basic code, one basic line at a time. It even gives me the option to see the assembled code between each basic line, and step through that, too.