It's a great tool for formative assessments. It's easy to create the quiz and I like the different quiz formats (multiple-choice, short response, ranking). You could even add images too which is useful. The space race function is cute and the kids enjoy it.
The only thing that's a little cumbersome is when you check your results on the "Launch" page. It's not very straight-forward and so it's a little tricky to run the results quickly. Also, it doesn't tell you how to share the quiz when you first start (you have the kids go to Socrative Students and type in your room name).
How I Use It
Socrative is a great tool to create quizzes. Students can take the quizzes independently in class or at home. They could also work in groups to take the quizzes. Students are able to access the quizzes quickly by typing in the name of your room. If you have slow internet and you want to have a live view of the space race, you'll need to refresh or you won't be able to see the most up-to-date results.