How to answer questions *honestly* about some professors from Visit Days students? 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.
First of all you should decide whether you want to convey any negative information, factual or otherwise, about these faculty members.  You are certainly not obligated to discuss the issue at all.  You could for instance contrive not to be present for this part of the Visit Day, or you could show up and say "I'm sorry, but I don't have anything to contribute in response to that question.  Maybe you should ask someone else."  

In my opinion though, absenting some very clear and serious negative consequences for you, you should provide feedback on your experience, because as you say,


  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.


You go on to write


  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. 


Telling the truth about your own experiences is not "bad-mouthing" anyone.  


  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. 


I suggest that you plan out what you will say, or could say, in advance.  Indeed you should concentrate on factual information.  You should also remind them that this is just your own experience, and that others' experiences might be different.  This should encourage the visitors to seek out information from others, which is good.

I also don't think "bias" is the right word.  Your answer to the question "What's it like to work with Professor X?" is inherently about your feelings on the matter.  If you had a negative experience, then that's the correct answer to the question as far as you can provide.  It doesn't mean that others will have the same experience -- and again, you can emphasize that in your response -- but it is your take on the situation.

What is the alternative?  Dishonestly describe your experiences and thereby lure further students into bad experiences?  That's not helpful to anyone.