I can't comment on much of his work, as I've only read bits and pieces. In general, academics I've met have varying opinions of him based on which books they've read, but he was certainly a major figure in the field of academic military history.

Keegan's work that I'm most familiar with is The Face of Battle, and it's still an excellent book, and a great introduction to military history. I would happily recommend it for people to read. While the discussions of the individual battles have moved on since it was originally written, for example if you want a dedicated work on Agincourt you should read Anne Curry, it still has its place on a historian's bookshelf. I particularly like the idea of taking three battles which feature a somewhat overlapping group of belligerents, and take place in roughly the same geographical area, and studying them together. It's an interesting approach, and something I'd be interested to see more of.

His other books I am far less qualified to comment on. I remember when his history of the American Civil War came out it was not well received, with at least one review I read criticizing him for relying mostly on outdated scholarship.

Given the sheer breadth of subjects he wrote on, it's inevitable that the quality of his books varies significantly (one can only master so many topics in a lifetime), and I suspect people's opinions of him vary substantially based on which of his books are relevant to his field.

I will also fully confess to being a bit of a Clausewitzian myself, so his A History of Warfare never really appealed to me.