Admittedly Messi was a little guilty of this in the match against Arsenal, in general I think he does his best to stay on his feet and if he does go down, he usually gets up without much fuss. Messi is certainly the best player about just now but I don't think you can really make the best ever comparisons until after or at least in the latter years of his career. He is a joy to watch though, I've been lucky enough to see him play a few times at Camp Nou and it's certainly a different experience than watching Mackie at Pittodrie!