Before doing my own, I asked on forums for opinions.  Overwhelmingly, I was told not to use them and to hand pack.  So of course, I used them.  The concern was, as mentioned above, blowing out the rear seal.
I, too, put many, many pumps.  I did so until it was obviously clean grease coming out of the front.  Key to not blowing the seals, is that I went very slow while my son kept the wheel spinning for me.  We spent about 15 minutes on each wheel (that slow).  
I was able to see all of the dirty grease come out until it was clean.  At that point, I had confidence the job was done.
I've towed about 10k miles since and the rear seals were fine.  I'm going to continue to use the system, but carefully.