If you search around, you'll find two different types of services:
1) Services that are oriented towards the SEO community, and are garbage. They either steal content and spin it, or use crappy PLR content and spin it. Either way, unreadable garbage that you don't want.
I've seen one that produced grammatically-correct content, but it seemed to do that by stealing source sentences from lots of different articles, and assembling them into one article. Grammatically correct, but not truly readable,
2) Services that are based on artificial intelligence, and are typically only useful (right now) in a very narrow context, and even then, with an enormous amount of expensive setup. For example, it is used with success in auto-generating textual weather forecasts, movie review websites, etc. Requires environments with rich and structured underlying, data. Also requires tons of setup for the verbiage variations.
There is an "almost automated" approach that would be much cheaper, and produce a broader array of content types. The most well-known "article writing service website" offers an api where you can purchase written blog posts and then publish them as they are finished. You can either write the binding code yourself, or if you use wordpress, they offer a plugin. Of course, if you have the articles going to an open pool of writers, the quality will vary.