I also tried GIMP. It’s decent for a free art program, but I wish the pencil wasn’t set to ‘n’ and the colour picker set to ‘o’. It made things really difficult, not to mention the odd way it tries to recognize pen pressure, which is completely velocity based. It’s easy to use once you figure out how to make the brushes do what you want.
Of course, my lines are always going to come out shaky if I’m not using SAI. And I got bored with this halfway through. it took much longer than it should have. I could have done this in less than an hour in SAI, but this here took me two hours.
In the end my oppinion of GIMP is very much an “I won’t use it unless I have to.” But it IS a free art program and I recommend it to anyone who can’t afford anything else.
It also takes a long time to open up.