There is a fair amount of sheer whimsy in the film--a characteristic Nouvelle Vague feature that sometimes works, coming across as fresh and unexpected, and sometimes sort of falls flat, seeming merely throwaway, unserious.
But the film as a whole I found fascinating and wonderfully suggestive, particularly its guiding metaphor. This compares wild grass--uncultivated, uncared for, appearing in and around things of more usefulness (a paradigm case insisted upon by the film's opening credits is that of grass growing in cracks of pavement and tarmac)--to stray relationships, equally unplanned and uncultivated, that can sprout up in the cracks of more ordered segments of life, out of seemingly trivial encounters.
The central characters in Herbes Folles are not friends, not business acquaintances, not neighbors, not lovers (even Truffaut's famous menage a trois in Jules et Jim has more solid substance than what Resnais offers up here, though clearly we're dealing with similar inspirations and approaches)--their relationship has no rationale, no name, so it has no rules or accepted conventions, either. This makes it fraught, susceptible to endless hesitations and uncertainties, second guessings and regrets--in fact its very pointlessness renders it curiously intense, there being no established roles available to fall back into in case of missteps, the way an unsteady acrobat can drop into a net.
In this respect, the film is actually about freshness and unexpectedness: these aren't just inherited tics of a French film school. I found Resnais here to be thought-provoking in the same way that Dostoevsky is thought-provoking: people and their relationships are shown as fundamentally unpredictable, mysterious even to themselves.