I think having a poker coach is an excellent, if not essential idea. You will not find a professional golfer without a swing coach, or a baseball player without a hitting instructor, so why should poker be any different? In a game that involves so much mental application, having a third party observe, analyze, and help your play is a very good thing.
The first thing most people look at is the price of a coach. Most coaches charge quite a high hourly rate. Why? Because they can make money playing poker, so if they are going to stop playing to teach, they have to replace the income they are losing by not playing. Most coaches charge in the $200-$350/hour range and that is a lot of money. There are some coaches around who are cheaper, but are they as good or do you get what you pay for?
When choosing a coach you should forget about the price. The price is irrelevant compared to what you will make over your poker life time if your play improves. What you must do instead is choose one that fits what you want to do. Some of the better coaches offer a short conference BEFORE you plunk down your money to see if you are a good fit for their coaching and vice-versa. In other words, you do not want a coach who specializes in cash games teaching you tournament play to try to get you to the WSOP. It is unwise if you want to be a Sit n Go player to pick a coach who only plays large multi table tournaments.
What I see a lot of, is coaches who play at a $100/$200 level teaching players who want to play at a $.50/$1.00 level and these are two totally different games. It is important to make sure your coach has actual, not theoretical, experience in the game you want to play. 6-handed is different from 9-handed in the strategies you will play. Multi-table tournaments require a much different style than cash games. Dollar level can be the most important factor. If your coach only plays high dollar poker games, he has no clue what goes on in micro games and shouldn't be teaching you. What if you want to learn Omaha or HORSE?
The bottom line is that you need to qualify your coach before paying him. Just because he is more expensive or has won some tournament somewhere does not mean he is the coach for you. There are a lot of options out there, seek out the one that fits you and then go from there.