The only exposure I've had to a bittering extract was in an "off-flavor" seminar. They used it to set precise IBU levels, so we could try to sort them out. All but one person got the order wrong. I found the bittering extremely harsh, but that might have been because the base beer (Coors Light) had very low IBUs. That meant almost all of the bittering came from the extract.
I have used a hop oil concentrate (hop helper) to fluff up an old IPA Brut. Not as good as re-dryhopping, but you can do it by the pint.