I agree. Keane was always the best player after Cantona retired and before Van Nistelrooy arrived. He was never going to be as flashy as Beckham and co, but the two aren't comparable in terms of impact.
Keane seems to be even more underrated than Scholes, in that regard.
As far as Beckham goes, he was exceptional prior to 2003, and was easily one of the best in the world. I think he might have been 2nd or 3rd best at the club at his peak, and retired having accomplished so much.
But some of the comparisons here (to DeBruyne for example
) are a little wild.