I normally handle my own campaigns through Google Adwords because the level I need doeswn't usually require outsourcing it.

However, I will say in general don't use the first company that comes along. Check out thier track record, try to talk to people that have used them (that they don't refer you to), compare rates and fees.

There are some good actors out there, but there is also a lot of room for smoke and mirrors, overcharging, and outright fraud in this sector.

Also, have a budget and manage expectations. Even the best out there can't help you without real budget and time to see results, adjust, etc.

When I worked with a firm that offered this as a service, I would stress up  and down until I was blue in the fact it takes a while and clients wanted to cancel and act like we were fraudsters after one month.