Heres the deal I have been spending six months on a site with 8k exact match keyword search which is not exactly a lot however I gauged the competition to be low with amazon pages and random forum posts at the top of google. Anyway I have put a lot of work into it and I have made pennies due to unstable rankings. At the other end of the scale I completed a mini content website for very low competition and very low searches. However this ranked all of its websites on the first page in google with no backlink building what so ever in about two weeks and stayed there. That website made a steady £5 every month which is not a lot but two days work can't complain. So whats the way to go here? The way I see it high potential keywords have a lot of risk. Whereas low ones don't but pay out sucks.
Both low and high competition keywords can work well, I prefer high comp because of the opportunities they can bring in terms of alternate advertising. I recently released and article about high comp over low comp if your interested: http://blog-newz.com/blog/why-you-should-target-high-competition-keywords/
I've never really seen anyone recommend going after highly competitive keywords and my experience speaks to this too. Health insurance is a good example where it is almost impossible to rank high enough to matter...getting long tail traffic just doesn't cut it in these situations.
Low or medium competition rate and high search result. "Is better to be rich and healthy than poor and suffering" Woody Allen ( I think).