Use consultants, but don't rely on them. They are wonderful people and deserve a great deal of respect. Since they get to see a lot of different projects, they often know more about specific technologies and even programming techniques than you will. The best way to use them is as educators in-house that can teach by example.
However, they usually cannot become part of the team in the same sense that regular employees are, if only because you may not have enough time to learn their strengths and weaknesses. Their financial commitment is much lower. They can move more easily. They may have less to gain if the company does well. Some will be good, some will be average, and some will be bad, but hopefully your selection of consultants will not be as careful as your selection of employees, so you will get more bad ones.
If consultants are going to write code, you must review it carefully as you go along. You cannot get to the end of the a project with the risk of a large block of code that has not been reviewed. This is true of all team members, really, but you will usually have more knowledge of the team members closer to you.