I was wondering if there were many users who wouldn't spend $19.99/month on a service when they only wanted one feature, and to see if they would pay $4.99/year for that one feature instead.

I am thinking about making it entirely free and using it to solely promote TweetingMachine. I might stick a TweetingMachine advert on it for a week and see if that results in any sign ups and go from there.

Thanks for your compliments by the way - I have a few "proper" failures I can write about in the future if you're interested. Although I'm also waiting for the magic $500/hour moment ;)