Easily the least impressive of the Mars books. The first story reads like a bad dream about Barsoom: absurdity on top of absurdity; poorly written; random; etc. The second story's villains are a nice addition to the universe, but they're wasted in an incomplete story that recycles pretty much every detail from earlier stories (invisible ships, Dejah Thoris in danger, yet another invasion of Helium, inhabitants of an invisible city, and so on). I did like that the intelligent plants on Jupiter have slow nervous systems and 'scream' minutes after you hurt them, and the description of the ocean was nice.
Here's how I'd rate the Mars books in relation to each other: 1 > 2 > 3 > 8 > 7 > 5 > 6 > 9 > 10 > 4 > 11. Books 1-3 are an essential set that should definitely be read first before all others. After that, the only sequences that really matter are that 6 should be read before 9, and 7 and perhaps 5 should be read before 10.