I posted about this before.. but I really don't understand the absurdly-high rating the double-Ambassador opening gets. Maybe it makes some sense when you're up against another player also using Ambassador.. but without an influx of Estates and Coppers to volley back at the opponent, I've frequently found that double Ambassador will whittle my own economy down too quickly and clog up. Plus in this case not only is his second Ambassador semi-useless, it also just increases the likelihood of you getting to use your Horse Traders (a card that imo gets underrated).
I think what it comes down to is, even though double amb is worse than single ambassador against an opponent who isn't ambassadoring, the card is so good that you usually end up beating them anyway, and double ambassador beats single ambassador. That said, someone on this forum was advocating opening amb/silver and then getting a second ambassador on turns 3 or 4, and I've found that I actually like that more than amb/amb on most boards.
On an unrelated note, the only time I can ever remember beating ambassador with a deck full of crap was
this game, where I fall behind on the ambassador war pretty quickly, and start buying green cards. I don't think my opponent played his best(if you can consistently draw your deck, it might not be time to get rid of all your coppers, for instance. Also, so many black markets), but it was still kind of cool.