Radio Adds through Google with $400 towards first add to get you started any beta testers can provide feedback on how well it worked for them.
It appears recommended minimum spend it $500 -- $1000 per week but can not seem to work out how many 30 second slots for that, also seems to be able to target well defined area's , would be very interested on beta testers feedback
I have been running a national campaign for about 2 weeks now. I am very impressed with how Google has set this up. The reporting is top notch. You get an email in the morning telling you when your ads a scheduled to run and then throughout the day you can see when the ad actually ran.
Seems to be very targeted so far, been getting a ton of plays on stations across the country (US).
Just like with anything else, the less you spend the worse the ads are. When I lower the cpm I end up with a lot of ads in Alaska between 1am and 3am. but Iím ok with that for testing purposes.
As for roi, not sure how that would be. I am only testing the ads on a pet project of mine and strictly for branding.