The app sits in your Menu Bar waiting to be invoked, or you can hit Command-Shift-5 to bring it up. When you have outlined your object, you click on the first point, hit Return and your screenshot is saved to a location you specify in the app's preferences.
Once your points are selected, you can modify them, or use the arrow keys on your keyboard to fine-tune the selection, either one or four pixels at a time. Any control point can be edited or deleted, giving you a lot of flexibility. The only negative is that all captures are saved as PNG files, the same format Apple uses. I'd like to see an option where I could save as a JPG, TIFF, BMP or GIF.
I tried the app with some difficult shapes, and it worked very well. I did have to refer to the built-in help to learn all the commands and key combinations, but it soon became second nature. In my work for TUAW I am constantly grabbing images like logos or screen shots of apps I am testing, so Screenshot FX is going to be a much-used item. There are similar apps around, like Screenshot Menu, which is free, but you can only draw a rectangle with it. Screenshots ($9.99) is also worth a look, as it has some nice features like sending directly to mail or iPhoto.
Screenshot FX is a handy little utility that I'll use a lot. If you find yourself needing screenshots, and want more functionality than Apple gives you, this app is just the ticket.
Screenshot FX needs OS X 10.7 or later, and a 64-bit processor.