Reviewing code with an eye for improvement it is certainly one good way to advance our skills as programmers, but I don't agree that it would be the most effect

Reviewing code with an eye for improvement it is certainly one good way to advance our skills as programmers, but I don't agree that it would be the most effective use of our spare time. The problem is that we would always end up looking at it through the prism of our present knowledge. For example, you could probably write a better XML parser if you were familiar with things like XPath than you could if you only knew how to iterate through all of the nodes. Instead we should spend our work time critiquing our code, and our free time learning the things that both interest us and have some practical application (reading sites like HN is a good way of gauging this). Some of this learning will naturally involve studying others' code.