we love our ergo.
we must use it at least three times a day, sometimes for two hours at a time.
every once in awhile I have it on wrong and it hurts my back...but that just means i need to adjust it.
i've never needed to take a diaper bag on my (relatively short-under three hours) trips to the zoo etc. the front pocket holds two or three disposables, a small package of wipes, my wallet, cell phone, keys, and a small change of clothes.
we have actually used the ergo outfacing, and have seen friends do likewise.
its not as secure, and baby is pretty much in the splits. but he likes it sometimes.