Zap

I've been waiting to try a GUI toolkit since I learned Windows API. For those that have used it: How is it? What can you do with it? What can't it do that API can? Is it worth abandoning API to learn? Since I found a GUI designer for it, I've been thinking about trying it. My major problem with tring it is: Will I not be able to do something in wxWidgets that I can do in API? Also, does it make programs too big? Because I also want to keep my programs as small as possible.