we just got ours last weekend. i've tried it with LO twice so far... no more than one hour. i was doing chores in the house.
pro: no hip, back or shoulder pain for me. LO seems to be ok with it. the insert does give a good head support for LO. easy to get sleeping infant out of the carrier. straps are adjustable for DH and me.
con: with the insert it feels bulky. a little bit hard for me to put LO in, but i probably just need more practise; after all i only used it twice. can't wear the carrier on quickly because of the safety feature on the clip for the hip section.
I LOVE my Ergo. I can't believe how comfortable it is.
We didn't buy the infant insert because we'd heard it's a waste of money. We wrap DS in a blanket (usually an Aden & Anais) before putting him in it, and it's not bulky.