Well some time ago one of my sites had a substantial PR boost 5->7 along with nealy dropping out of Google's index. I mean in one day(it was the day PR update occured) I got PR 7 and traffic from Google stopped at all. some time later PR went back to 6, but traffic didn't return. Sh*t happens. I guess this was due to an enormous link developing activity
Pagerank does not guarantee web traffic but it is a good thing to have pr6 and above. For better understanding, please read http://www.iprcom.com/papers/pagerank/