LucidSW - 2:41 pm on Mar 30, 2011 (gmt 0)
If they act as a manager of your account and you get a competent person to do the job, it should be well worth it in my opinion. I've taken many accounts myself and improved efficiency by well more than 20%.
I assume that 10-20% is percentage of spend. For how long? Just make sure you understand any contract and the metrics used. A couple of times, I've had hard time explaining to a client that their ROI is improved. All they looked at was a reduction of the number of sales (or clicks or impressions) when they were in fact making more profits, a better ROI, which to me is the bottom line.