Though I can't say that I have really had a "bad" experience an any of the labs I've worked in, I've certainly found that some go better than others. If you are in a bad situation, find some way to gracefully get out of it. I think that you'll find, particularly when it comes time to actually get your PhD, that what matters most is learning good science, not the particular project you are working on or with whom you are working. I have found that if you are having a bad time in the lab, you won't be learning as much as if you were enthused to get in there each day.
Now, as for the first questiono as to whether you _should_ do it, that one is more difficult. Idealy, I would say that if you are not passionate about doing MD/PhD, then you had best leave it alone. It is a long track and probably not worth it for a few extra letters in your suffix if you won't love what you do when you're done. While I have known some who at first decided to just go the MD track and later changed their minds when they found their niche (usually in a completely different field than they first thought), that is probably not the norm. Another thing to consider is that some MD programs are geared with research in mind, such as Stanford which offers the possibility of a 5th year for whatever (and the new curriculum that is being developed will in fact make MD research even more mainstream), Yale which requires a thesis, Duke, which devotes the third year to research, etc.
