Spy is fun but not useful. For example, the keywords it finds are useless if you want to find something to help you improve your own. It ALWAYS finds (for example) that the keyword BOOK is fantastic. Really? Maybe it because EVERY bloody series on amazon uses the work book as part of the series thing (series xyz BOOK 2) So where does the fun part come in? It also calculates how much MONEY the title is bringing in. Some of the trad books are absolutely eye popping, I can tell you. We are talking big money.
Samurai though is serious stuff. It does crash sometimes, but its worth the money for the time it saves. ALL it does though is analyse search terms that someone might type into a search at Amazon. You CAN do the same thing by typing in the keywords on amazon, and writing down the phrases that Amazon offers. From those you can see what people are choosing. Samurai does this and goes further by displaying average rankings linked with those keywords, and the number of searches being run using them.
Once you have learned what keywords you like to use for a series though, you'll find you won't use the software anymore. All my series are established ones. I just copy paste my previous keywords for a new book. So it WAS useful once, but unless you have an entirely new series to research, I doubt you will use it very often.