Keyword research is an activity you undertake every now and then. If you have a clear definition about the product or service you want to ‘sell’ with your website, you should be able to come up with keywords, related keywords, and even more related keywords to make your awesome website (more) findable. As your product and the market will evolve, your keyword strategy should do the same.
So whilst Long Tail Pro has just continued to grow in revenue every single year and comes highly recommended, you may want to check if all the important features are fully functioning before signing up. Especially the KC feature (keyword competitiveness) as it is so fundamental to keyword research and making an informed choice on whether or not to go into a particular niche.