I have been writing at Helium for about a month now and I see both its potential and its limitations. I have seen both positive and negative reviews of the site as a writing site and I think that each argument has some merit.
For instance, as far as publishing my writing on the web, I am perfectly capable of doing that myself. I should probably do it more often as several of my blogs are slightly behind on the updating schedule. They do offer payment in the form of a percentage of the advertising revenue. However, I have my own advertising revenue of which I get the entire payment. What they may offer in this area is wider exposure and possible higher ranking in the search engines.
What they do offer that is a boon for those who are considering freelance writing is an opportunity to submit articles for review to publishers who will pay for the article if it is the one chosen. It's a bit like a cattle call, in that many other members will be writing for the same opportunity, but that makes the selection of your article by the publisher even more satisfying. You have to beat out a lot of other articles to get noticed. Additionally, it's a place to get started in the area of freelance and build a portfolio of work.
That being said, I am not straying from Helium anytime soon and have begun to concentrate on the publisher opportunities rather than any other venues and channels. I have had some success here, and have sold several articles.
What anyone gets out of a site like Helium is what they put into it, but they must know what it is they want before they start. For a blogger, the prospect of writing just to see it on the net is not enough to warrant putting a lot of effort into what is essentially, someone else's site. But if a blogger wants to enhance their resume and broaden their experiences by learning to write on someone else's demand, it's a decent training ground.
If anyone is interested, my Helium pages can be found here. A list of my articles can be found here.