We are having Visit Days soon, and we (current grads) are expected to have one-on-one conversations with visiting students with research interests closest to ours. One typical question we get, "How is Prof. X? What is it like working with them?".
As a current grad, I can see the names of the visiting students along with the professors who are their top choices for advisor, and several students have listed two professors I've tried to work with in the past (they had been my top choices when I joined this university).
These professors are great researchers, but, in my opinion, not good at all as advisors. I am not going into details because that'll end up being a rant, which isn't the point here. But my view is strongly shared by all my group peers who had, at some point, tried to work with these professors (due to their fame as researchers).
Because nobody had warned me before I started working with these professors, I ended up wasting two years in grad school, in my opinion, a very, very high price to pay. I would like to be very honest with the visiting students. At the same time, my field is quite close-knit, with all top professors collaborating with those at other places. If these visiting students end up not joining us, I'd not want them to remember me as the person who bad-mouthed these famous researchers.
How can I honestly answer questions about these professors without coming across as bitter (which I admittedly still am)? If I just state the facts, well, even those are going to be coloured by my viewpoint, and will therefore be biased.