I think it depends on your site. I have some sites that consists only of images and meta tags are very important for sites like that same as alt tags.
someone told me before and it's good advice: "it doesn't cost you anything to put up good meta-tags - whether or not they are useful - i.e. you've got nothing to lose..."
Superclick, Do you see an actual correlation between your meta & alt tags and the type of ads you get? I was looking at one of your sites & seems like the ads are prompted more by your URL than your alt or meta tags. I’m wondering because I’m in the same boat as you; a niche site whose content is mostly imagery.
I think that just a little, specially description, keywords not at all IMO, so with keywords be careful, not to much...
I think Google takes the overall theme of the page and provides all varying ads around that theme. However, sometimes the tragetting is rather poor.
Good Page Title and Contents are countable, google didnt use meta tags now , its an old story right now.