I won't summarise the plot. Other people have done that superbly. What I'll offer instead, is my general, uninformed, unexpert take on the book. As someone who's read a reasonable amount of Pynchon, but who is - by no means - a Pynchon fanatic.
I like Pynchon. A lot. I love the depth and complexity of his writing. I love the feeling of ploughing through a deep, rich, fertile text absolutely jam-packed with Significance. Replete with allusions, half-allusions, hintings, suggestions... Comments that you'd need to go 400 pages back, in order to recall the full significance of the full in-text meaning... without beginning to consider what they might otherwise mean in a broader, deeper, fuller context...
I love the fact that I've given up on most Pynchon books at least once. But have always been drawn back to them. Wanting to read them, understand them, approach them, immerse myself in them... to understand at least some of their meaning. With the hope that a second, third, fourth reading will uncover another layer, and another layer, and another layer... I have started Gravity's Rainbow 9 times, and got to the end on three. That isn't because it's a bad book; it's because of the layers of flowing, suggesting, rhythmic density have lost me sometimes... (Usually, fwiw, around the episode with the church / singing in an English midwinter...)
In Inherent Vice, all of that is gone. All of it. What's left reads - to me - like a juvenalia fest. Strip out all the effort, depth, complexity and difficulty from a Pynchon novel, and what have you got left...? The answer seems to be - as someone else has suggested - something remarkably close to Elmore Leonard. A neat, linear, eminently followable storyline with a manageable quantity of characters. All of them behaving slightly zanily, but nothing... Slothrop-esque. There's a conspiracy. But it's not a particularly well-drawn conspiracy (or maybe it's too well-drawn...). Nothing subtle. Nothing half-hidden. Nothing hinting from the shadows, full of murky and sinister suggestion. Just 'ooo, there's this thing called 'Golden Fang,' it keeps on cropping up all over. Ooo.'
Don't get me wrong, it's not completely unreadable. It's lively enough. Stuff happens. It plods along at a reasonable pace.
But I keep on getting this feeling of 'why am I bothering?' This is not the Pynchon I know, love, or appreciate. It's Elmore Leonard through and through. Bish, bash, bosh. Sorted. Yeah, there's a market for Elmore Leonard, great. Fingers crossed, this will - indeed - introduce a new generation to Pynchon. Though god only knows what they'll make of something like V, Mason and Dixon or Gravity's Rainbow after being introduced to his work through this great clumsy lump of Obviousness and Unsubtlety.
But... as someone who wants to read Pynchon because he writes in a different way... to be lost, amazed, bewildered, amused, perplexed, delighted, confounded, confused... this just is not it. Not by a long stretch.