I don't want an ad network that puts you anywhere. I don't want dozens of ad sizes and specs to worry about.
I could really use an ad service that let's me easily buy and manage the basic blog ads across dozens or hundreds of targeted blogs. Otherwise, we have to buy and manage each blog directly meaning I can handle maybe a dozen before the time spent really weighs on the ROI.
Check out BuySellAds -- ad sizes are pretty uniform, and you can hand-pick targeted sites where your ads are displayed.
You can also target display ads within Google AdWords if you put in the time, and AdSense ads are constrained to a specific list of sizes.
I'm gonna go out on a limb here and say... you are approaching advertising in the wrong way.
Assuming that you're looking for ROI from adveritisng, the formula is very simple:
Instead of looking for "dozens or hundreds of targeted blogs" look for a small handful of large sites (three or four tops) and invest in A/B (and even A/B/C/D/E/F/G) testing on different banner designs, landing page designs, etc. Once you find the sites with the right CTR and CPM, just keep throwing more money at them until the ROI starts to drop off.