It might be difficult to measure evolutionary progress, but that is no excuse for ditching the whole concept:
Evolution is an optimisation process—similar to a genetic algorithm. We even know what the fitness function is—and so can see what is being optimised. Optimisation processes are—by their very nature—powerfully directional processes.
There is a way in which such a process could fail to be directional—if it had too large a mutation rate. That might happen if our planet was extremely radioactive, or if it was frequently bombed by large meteorite strikes. However, in practice, neither of these things applies.
A brief examination of the history of life shows that it is characterised by an accumulation of “survival technology”—i.e. either adaptations or technology.
In particular, the biosphere is accumulating “natural technology” that helps it more rapidly identify sources of potential energy—and degrade them in the process of constructing offspring.
The accumulation is progressive, cumulative—and inexorable in character—and the resulting ratchet mechanism provides evolution with a powerful progressive, directional character.
It might be difficult to measure evolutionary progress, but that is no excuse for ditching the whole concept:
http://alife.co.uk/essays/technology_explosion/
http://originoflife.net/direction/