Experts?
We've never done this before. Who's an expert? How often have we been told by scientists that "this is absolutely safe", only to find that it kills us? X-Rays, DDT, DES, Thalidomide, and on and on. What scientists say is worthless.
Having said all that, the LHC isn't going to be doing anything that hasn't been done before. It's just going to let us have a closer look while it's happening. The black holes and strangelets and such aren't even theoretical. They're wild imaginings. Collisions of particles at these energies happened in the past and still happen. Somehow, the Universe manages to get along.
What I *would* consider more of a potential threat is the possible harm from radiation coming from this device. We don't know what that might do. I suppose it might be a new danger, though that seems unlikely.