I have used Spyfu and other similar tools for years. They do provide a little bit of insight into what your competitors are doing, but I should emphasize that it is just a hint, definitely not a complete and accurate picture. All of their data is extrapolated from a relatively small sample set, it is incomplete, and often highly inaccurate.
The data provided by such tools pale in comparison to what you can get from your own direct testing of keywords. “Seeing what are the most lucrative keywords from your competitors” is easily and more reliably achieved by using the AdWords Keyword Planner tool. Just look for keywords with relatively high avg, CPC, and high competition level, and you have that list.
What I find most useful from the data those tools provide are the ad text samples. It can give you a fair amount of insight into which value propositions your competitors are using in their ad text. You can get that data from the AdWords Ad Preview and Diagnostics tool, but using Spyfu, or similar tools, will be a bit faster. Finding a good value proposition that your competitors are not using effectively can be worth quite a lot if exploited properly.