What are the advantages and disadvantages of using a professional advertising agency in promoting a product?

Advantages
They have creative and artistic resources, they can provide access to these resources more cheaply or for short term requirements, they provide an independent view of your product, they may have better information on demographics across a broader region of the country or internationally, they can normally buy advertising at rates lower than individual companies can, they give you someone to sue if it is screwed up.

Disadvantages
They take things out of your control such as delivery schedule, they can be more expensive, they can have a conflict if they also advertise for your competition, they may not fully understand your product value or your customers as well as you

Any ad agency worth it's salt will know your product as well as you do. They will know your customer as well or better than you do. It's their job. It's why you hire them, to know more about who you're selling to and how to sell to them than you do.

There is no disadvantage if you're serious about selling product.
I mean, you can represent yourself in a murder trial but don't expect the results to be as good as they could be. Same thing with advertising.

Pros
Expertise: The RIGHT agency will know your target and how to best communicate your product's benefits to them. They'll know your challenges and your opportunities.

Cons
Cost: Ad agencies are an expense/investment. To some, a "con" would be the fact that they'll have to relinquish some control over the marketing process.

Basically, I've never worked with a successful client who was sorry that they had employed an ad agency.