For the moment, we only use it for syntax analysis and to give a beautiful error output for our users. There is an active LLVM backend for AVR with great ponential, currently under development and from what i understand, Atmel is backing it as well, but it's not stable yet. We would like to try it out though.

P.S. A size comparison will be very interesting. I will go out on a limb and assume that LLVM spits larger binaries when optimizing the code for speed. We use -Os to optimize for size, so it will be interesting to see which does the best job.