I have had the privilege of mentoring three students since I started working. While others I know have had bad experiences, working with these students has been a very positive and fulfilling experience for me.
Of course I was careful to select only the best students to hire, but I think that the main reason my experience has been so positive is because of my attitude going into the mentoring relationship.
I see an internship as an opportunity for an up-and-coming scientist to learn science, not as a way for me to get some extra work done. And because I have that attitude, I'm never disappointed.
I fully expect my students to be time sinks, and I have developed ways of mitigating the worst wastes of my time; for example, I provide them with a handout detailing the project parameters on the day they report, and I schedule weekly meetings with them and try to touch base briefly once a day. They can read the handout rather than having to rely on any notes they would have taken while I was verbally explaining the project, resulting in fewer gaps in their understanding. I schedule for one hour per week of my uninterrupted time, and I usually go touch base with them about 15-30 minutes before a meeting, allowing me an excuse to escape if my time is being wasted.
I also assign them a project that it would be nice to have completed, but that is not vital to my work. In this way I can be only pleasantly surprised by their progress on the project, rather than depending on their work.
So far I have not been disappointed. Even if my student gets nothing tangible done, I do not consider my time to have been wasted. Students don't learn high-performance computing in school; it's really something that has to be learned through experience, and that is what I have enabled them to do during their internship with me. And if we don't give them the chance to learn it, how are we ever going to be able to develop the next generation of computational scientists?
I was once a student who knew nothing about high-performance computing. Someone took a chance on me and I was hired as a graduate research assistant in our university's HPC center, despite my lack of knowledge. It took some mentoring and time-wasting on his part, but I learned the skills that I needed to get to where I am today. I think it's only fair that I give others the same opportunities that my mentor gave me.