I have experience using it both as a publisher and an advertiser.
Publisher: It did not work out for me, as most of my sites are around a niche topic, which does not seem to do well for contextual marketing (but does well for affiliates). I know of some other publishers who do well with AdSence and Vibrant. From what I have seen and heard if you have acres and acres of diverse content (e.g. not just widgets/blue widgets/green widgets but widgets (all colors) doodads (all shapes) and thingamabobs (all types)); and very long text rich pages you can do well. If not do not pass go do not collect $100.
Advertiser: Very mixed results, sometimes would work out…but many times I would find my ads showing up in negative articles (e.g. Forbes.com has an article about how blue widgets are nasty, ‘need blue widgets’ appears in the story and is hyperlinked to my website) this would more than cancel out what I was looking for.