as I replied to someone else... If you are going to use any of these services, it would truly be a waste of money to try using it on anything but your best quality posts. I'd even posit to say every post you make should be terrific and well thought out in it's own way.
This is your true calling card, and any attempts to attract more attention to yourself for mediocre content will likely attract little more than cold disdain and few (if any) new followers (and possibly even some a bunch of unfollows)!
So yes, I definitely agree with what you wrote: "we have to use them wisely and carefully."