It is very important that you do a thorough study to identify a niche [Thorough Study is not equal to Final Study].
Very true. But remember, In these years google has build one of the robust and powerful adwords tool. It will be similar to a cake walk on the other ppc tools if you thoroughly understand adwords. My recommendations is to select a niche which is more of unique and does draw lot many advertiser plus give you good returns. In Adwords terms, select such keywords that are very niche has low impression, high clicks and good ctr plus very relevant traffic. (This is something you can do after research and analysis)