As a user... your mileage WILL vary. It really depends on how you use the systems. If you focus on doing the work yourself, your account balance will barely take a tick upwards. Earning a few pennies a day is a great way to fool your mind into thinking you are being productive.
However, if you create a system where you continually funnel new leads into your GPT referral link, you have a great chance of having some solid success. I know some users who are stay-at-home moms/dads who are generating a very nice supplemental income from GPT sites. And good for them, they figured out the system!
A better chance of making money is a GPT advertiser. I've used a few paid-signup campaigns to get users to sign-up to my list or to other referral links. It has worked very well.
And of course, the best way of making money with GPT is to own a site yourself. If you are an admin, you actively promote your site and your users, you'll make a very decent living. It's very possible to make a full time living with a GPT site.
I was never a GPT owner, but I did own a traffic exchange for a while several years ago, and there was a crossover between the two types of sites.