I think it depends on your financial resources. If you are able to run through the patent process and even to bring your patent to market or at least find a licensee it would not make much sense to involve an invention marketing firm (their services cost money as well!).

But if you are not able to take the financial effort marketing firms seem to be a good help. I would choose a service where you have to pay a percentage on licensing revenues instead paying a fix fee. This ensures that the company really tries to sell or license your invention.