It does make the Ergo a bit trickier to put on, for me anyway. The first few times, I thought I was going to drop my baby. When I put the Ergo on, I glide my hand up and over the body of the carrier and so the backpack gets in the way a bit. You have to change your method of using the Ergo to use the backpack.
I prefer it to the pocket on the Ergo since if you put anything in the pocket, it's pretty much right against the baby. It is surprisingly huge and there are two ways to access stuff so that is very convenient. And it can be worn alone as a backpack, which is super handy for when there are 2 of you and 1 kid or if you want to do a front carry.
Unfortunately, I never really was able to put it through its paces because I didn't start using it until my baby was out of diapers.
I like it as a standalone backpack well enough, but I have had HUGE problems getting it on when attached to the Ergo. It doesn't attach well enough to the Ergo to keep it from dragging down at the bottom, particularly if you have anything heavier than a diaper in it, IME.
Plus, I like to be able to get to stuff without taking the whole contraption off, so I like a separate bag better.
I have heard people say they liked it better wearing it on the opposite side of the body than baby, so one in front and the other in back (although this you could do with any backpack).