If your 'case' is complex or marginal then a UK immigration advisor might offer some advantage to you.
Also if you really feel you just cannot give the necessary time to prepare the application yourself or don't feel able to do it then an advisor can again be a good resource.
The OISC (Office of the Immigration Services Commissioner) has a responsibility for the regulation of Immigration Advisers. They make sure that the advisor is a fit and competent professional who can act in your best interests.
Do make sure the advisor you choose is a member and is experienced. Try to find one by the recommendation of others. Also do be sure that whoever you take on has experience with similar application cases to yours.
There's a lot of people out there just waiting for your money. There are no short cuts or guarantees for a visa
The large majority of people do their own application and certainly those here on the forum have a great record of success.
Always remember immigration advisors cannot do all the work for you, they cannot collect all the documented evidence. You still need to do that yourself.
All they can do is ask you to supply information and the UKBA required documentation.
Let me suggest you review this recent forum thread
All New Guide - How to apply for a Spouse/Partner Visa
It may help in your decision making.
Take your time and feel free to ask any specific questions
