Yeah, he probably is. Good article in today's Observer about the man whose record he beat; http://www.guardian.co.uk/football/2012/dec/08/gerd-muller-lionel-messi
Not the greatest yet imo. He needs to keep at a high level for a few more years, or face a bit of adversity. I reckon Barca will hit a lean streak at some point in his career there (assuming he doesn't move). If he's still banging them in for fun and leading them to trophies whilst that happens then he'll be the greatest ever.
Messi is the greatest player of this generation so considering most players of this generation under current rules would make laughing stocks of players from yesteryear he has to be considered the greatest. However he could do with moving to another club, winning a world cup or at least having an impact at one and not retiring in tyhe next few years which I think is possible. These Argie bargies dont last long, especially the very talented ones. They tend to either go home to Argentina and be forgotten about or lose the plot. I think in a few years Messi will lose everything and become just some player but right now hes unstoppable.