We've used this service regularly and find it effective.
The key for anyone considering it, is to first determine your ultimate goal: sales, traffic, or followers. Once you identify your target goal(s), then you simply match the people doing the tweeting with your desired demographic. Not all models have the same demographics of users. So, it can be a bit of a trial and error process. But once you reach the optimal balance, the results are there.
Nice service Devin
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