So I've worked in this lab since about September 2010. I've spent the overwhelming majority of my time working under a grad student and I haven't really gotten to know my PI very well. He knows my name, occasionally talks to me about my project and classes and jokes around with me in lab meetings but besides that he's a really, really busy guy. I feel like he would only be able to write me an average LOR (whereas the grad student could write me a great one. obviously he can't actually write me one). I know I could get three good LORs from teachers, but how would it look if I didn't get a rec from my PI even though I worked there for two years? Would the whole "I mostly worked under a grad student and didn't get to know my PI that well" even be a plausible response if it came up in an interview?