I dont think most of us med student gets the best grasp on CTs and plain films during school..... both of which are much more important that U/S as far as grass roots goes.
I think U/S is important, but again its very dependent on what field you are going into. Like many other things in medical school, if your field or your interests permits wanting to know more about something, then spend free time learning about it. U/S is an increasingly important tool in Emergency Medicine...I knew that, and thus spent much time getting out of something else to go play with the U/S machine. Anytime we had a trauma patient or about any patient in the ED while I was on Surgery, I would ask if I could stick back and U/S them. I never once was told no that I can recall, I got to learn a bit more about the machine, and I missed out on more than one scrub opportunity (which was a plus to me). I did two weeks of OB U/S fourth year...just hanging out with techs, but I did learn a fair amount.