I think I would check it for the infamous clicking noise when you zoom in and out first. If it does it and it bothers you I wouldn't go any farther and send it back.
Second common problem that I hear about is the AFMA not being consistent from one end of the lens to the other. For example, and I do not have the exact numbers, I had one that was +1 at 24mm, about the same at 35mm and 50mm then took a dive at 70mm.
Some of the copies are soft at 70mm. At 24 and 35 it should be one of the sharpest lenses you own. If you own a 70-200mm II and it is close to or better than it at 70mm I would call it good.
Once you get a good copy you are comfortable with, it is a great lens.
For me the final lens requirement I was looking for was as good or better than my primes at 24 and 35. At least equal to my 70-200mm. Consistent through the range for AFMA. Of course I checked out the usual things like centering and such also.
Good Luck