In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).
I was going to post screenshots here showing my numbers for the past year – but the dashboards in the old affiliate platforms (Clickbank and JVZoo) don’t provide the best visuals – so I’ll just show you my total affiliate income for the past year. In one year (March 13, 2015 to March 13, 2016) I earned $3867.60 in commissions for promoting Long Tail Pro. Not bad for having just a mildly popular ‘passive income blog’ and only a small email list!
I’ve found google trends to be an interesting way to see if a keyword (and by extension a niche) is growing or shrinking, and whether it’s seasonal or not. I can’t think of any other tool out there that can reliably tell you this information, so that’s really useful. Also, if you’re building a site, especially an authority site, getting onto something that’s trending upwards is a fantastic idea.
The limit on manual keywords could be higher.  I personally wish we could input 10,000 keywords at a time, instead of 200.  However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input.  Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).