SEMrush is a very useful tool for both researching competitors when starting a site or for growing an established site. I really like to find weaker niche sites that still seem to be ranking for lots of keywords; SEMrush helps me see what they are ranking for and what I can potentially target. You can also see what keywords you’re on the cusp of ranking for with your established site - another very useful feature.
Using the good ol' free Google Keyword Planner is great. But there is an alternate universe of Long Tail keyword phrases that live in Google's other database--namely, their "Autocomplete database." That's the database that is accessed when you're typing queries into the Google search box (notice that list of suggested keywords that pop up as you type?)
For example, assume your search ad generated 5,000 impressions in one day, of which 100 visitors have come to your site, and three have converted for a total profit (not revenue!) of $300. In this case, a single visitor for that keyword is worth $3 to your business. Those 5,000 impressions in 24 hours could generate a click-through rate of between 18-36% with a #1 ranking (see the Slingshot SEO study for more on potential click-through rates), which would mean 900-1800 visits per day, at $3 each, or between 1 and 2 million dollars per year. No wonder businesses love search marketing!

So what exactly is Keyword Competitiveness (KC)? It’s only the feature within Long Tail Pro that completely changed the game and made keyword research so ridiculously simple that even a dumb truck driver like me could become an expert! It basically works like this. As soon as you click the button in Long Tail Pro Cloud to retrieve keywords, it automatically calculates a number for each keyword (between 1 and 100) based on several different SEO factors. Lower numbers mean the keyword is easier to rank for and higher means it is harder to rank for. More on that below…
Basically, Google shows the autocomplete suggestions whenever you start typing anything into Google search box. It is in Google's best interest to show the most relevant keywords in the autocomplete suggestions. Keywords that would help Google to retrieve the most relevant websites and help users find the most relevant content for their search query.

Search Analytics can be found under the 'Search Traffic' section and provides details of the keywords that drove clicks to your website, based on data for up to the last 90 days. The difference with Google's version is that you can filter the data to put extra context around the keywords, such as filtering by country to see keyword popularity based on country, which can be useful when carrying out keyword research for websites that service more than one country.
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.