I'm a huge fan of his films, he was a great actor and as a director he was pretty much the godfather of the American indie movie starting with Shadows in 1959. He's up there with my favourite directors.
I really don't think there is anything pretentious about his films, considering how influential they were on modern cinema. They are thoroughly heartfelt and a unique artists vision of human relationships and struggles. The only film which is slightly akward is his first as a director, due to an inexperienced cast. After that, especially starting with Faces which is about a marriage imploding over the course of one drunken night, his films are full of powerhouse performances. They are very much actor lead films as is understandable for an actor/director. The style of the films is in service of the improvisational performances, cinema verite, caught on the fly with lots of handheld camera. His films become more refined though and by the late 70s they are exquisitely shot. If you aren't into character studies and acting then his films may be too long for you, but I tend to find then riveting.
He made the occasional studio film of which Gloria, a drama about an ageing gangsters moll saddled with a kid who is the only survivor of a hit on a family, is probably the best (don't mistake it for the terrible remake with Sharon Stone). My favourite films of his are the ones he made with his wife Gena Rowlands, maybe the greatest and most under appreciated actor of American cinema of the 70s and early 80s. There is a sort of trilogy of films where the central character, always played by Rowlands, deals with mental illness or a mental breakdown of some sorts, A Woman Under the Influence, Opening Night and Love Streams. There is no self-pity or sentimentality about them due to Rowland's toughness as a performer which counteracts the vulnerabilities of her characters. All three films suggest that her "madness" is an integral part of her personality and that she isn't so much a victim as misunderstood.
That said, I think every film of his is worth checking out.