Well, Montaigne' Essais are a hodgepodge of philosophy (as Renaissance men understood the term, so including natural philosophy a lot), maxims, daily life observations, political or ethical short pieces, opinions on everything and nothing etc. The language is fun, but Les Essais are not especially noted (or read) for their literary quality (nothing like Montesquieu, Rabelais, Descartes or Saint-Simon for e.g.)

It's an interesting read for those interested in Renaissance thinking/non-fiction writing. Montaigne scattered himself a lot though, there's nothing systemic in Les Essais. It has some interest to those wishing to explore renaissance French (he's still more readable than someone like Rabelais, who was messing up a lot with the language on purpose) - but in the original it's not an easy read even for the average native francophone. Archaic spelling is fairly easy to get around, but the semantic shifts and the pre-Baroque grammar/syntax make a lot of sentences obscure at best without referring a lot to a renaissance dictionary. In modern French (most editions are translated to modern French) or another language it's far less interesting and I'd say a collection of the best essays by Montaigne is then far more recommended than the whole thing.

I liked Les Essais in small doses at a time, but in the end I prefer Montaigne's travel diary (narrating his trip to Italy going through the german states for the first time). But it's more akin to say, Samuel Pepy's Diaries, i.e. it's big collection of fascinated observations about daily life.