Yeah
Sometimes 18 nukes in a row get blocked, however, much more often, zero, one, two, or three get blocked. This war my SDI blocked 15/21 nukes, which is about 71.4%. Generally, when you put the data from a couple thousand nations together, it is pretty close to 60-40.
Edit: Statistically, just under an average of 2.5 nukes should be blocked before one hits.