I've been working on a research project this past few months under a professor's lab, but I've been mostly working with the professor's PhD students and other research assistants. I don't really meet the professor and have only talked to him basically once (at the start of the entire project). Is it a good idea to ask for a letter of recommendation from that professor? He's familiar with the overall output of our work, but he doesn't really know me on a personal level.
You can and should ask, but you could first ask them if they know you and your work well enough to write you a good letter. If not, ask them for advice.
I assume that the prof will probably ask the RAs who know you best for advice on the letter, so make sure you have a solid reputation with them, as well.
It isn't especially important that they know you on a personal level, but they need to be able to speak positively about your work and your likelihood of success.