"I've used many keyword tools over the years and Keyword Researcher is up there amongst the most useful of them. Apart from being a great way to get ideas for content, as an AdWords consultant, I also use it to uncover words and phrases that my clients would not want to be bidding on (negative keywords). This is a real time saver for me and a budget enhancer for my clients! All in all, this is a great app and should be used by any professional search engine marketer."
The WordStream suite of Keyword Tools is an integrated and actionable set of keyword tools designed to help search marketers with keyword search, keyword suggestion, keyword grouping, PPC keyword research, keyword analysis, long-tail keyword research, and negative keyword discovery, going beyond the capabilities that a typical free keyword tool can offer.
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.