Helium actually does have flat fees once you have writing stars. They are small (I believe it is 50 cents for each star you have). Since they take anyone and let you post anything, my account there is under a very generic pen name. You have to have a rating star to accrue ad revenue, so if you stop being active on the website you will not earn any more money.
That said, there are some benefits. Because they pay based on PVs, using Stumble/Digg/etc to promote your writing can actually increase your pennies pretty quickly. I would focus on Marketplace titles to try and earn extra income. They pay a flat fee and if your article isn't chosen, it transitions over to the main website and earns the upfront payment and revenue share.
Helium always pays on time and as promised. I don't actively write there, but I do keep my account active by rating and occasionally write to a Marketplace title.