Well there has never an instance in the series where the eighth gate has been reached, so no one has a real idea, but there's no doubt that it would be truly awesome. On Narutopedia (my bible at this point in my life), it says that opening the eighth gate gives power greater than the five kage, but despite this, again, it's hard to say. Remember (*SPOILER ALERT*), Madara defeated the Five without too much long-lasting damage to himself. I don't really know about Tobi's true power level, but it seems not to compare to Madara's, so that's a plus. xd
The bibl-site also said that Lee could only access five of the gates, however, since this was only shown during the Chunin Exam Prelims, we have no idea how many he can do. Plus, if anyone's going to use the eighth gate, it'll probably be Guy; he'll most definitely find it necessary to save his students and officially pass on his Will of Fire to Lee, or to save Kakashi while said silver-haired pervert has an epiphany about grief or something majestic like that.
You know, I love that word: majestic. You could be talking about a horse or watching your favorite anime's finale with manly tears of joy rolling down your cheeks (I AM a girl, despite Fabio telling you otherwise) and it all can be summed up with one righteously empowering word. Majestic.