I switched my google adsense ads from ad units (728 by 90) to 5 link units (728 by 15) I still get a similar amount of clicks, but they are worth nearly nothing. Does the value go down when you use link units? Just to note that its the same keywords that I pay $3+ for when I use adwords, but I only get 3 cents per click.
google adsense is not paying out like it use to, I think. I know that keyword payouts are always changing, but it seems that since many many people have jumped on the google adsense bandwagon, payouts and click though amounts have nose dived.
My experience, for my site, has been that average payouts for link units is much lower than for ad units, even though CTR is higher, resulting in lower overall revenue. However, your mileage may vary.
On one of my sites I make an average of .09-cents a click. Not great. In comparison, the banner ads make around .30-cents a click on the same site. But the difference is that the links get clicked more often, especially if you put it at the very top of the site underneath your site's logo. That tranlates into more clicks + more money via the links. They may give you less payout per click, but they do generate more clicks, so in the end that means higher earnings. Added: Plus, if you make the color of your links the same color as the rest of your site's links, then it doesn't stand out like a giant leaderboard banner. Saves you space and looks nicer IMO.
The reason for link units to pay a lower CPC is probably that more advertisements are shown, so the chance of a user clicking one of the lower paying ads will be much higher.
You do know that you could have both a 724x90 ad unit and 728x15 link unit on the same Webpage? So, it's not really an either or. I often put the link unit at the very top of my page, the leaderboard at the very bottom and a couple in between.
That gives you 3 ad units of various sizes, 1 link unit and 2 search boxes. Link units in general pay less than regular ad units. The beauty of them is that you can place them where you won't have been able to fit a regular ad unit. Also sometimes you do get ads from a link unit that pay more. It will depend on your site though.