A university research confirms the popularity of Hudson
With the usual caveat about the sampling size and geographic distribution of respondents, this paper found that the most commonly used CI tool today is Cruise Control (29 people), followed by Hudson (24 people.) The third popular tool only has 4 people, so beyond that is within a margin of error.
Now, if you look at the download statistics of Cruise Control (sorry if the link shows an error — it times out rather frequently), which reported 1830 downloads last week, and compare that with the statistics for Hudson (which reported 2868 downloads last week), you can see that Hudson is getting more downloads now.
Once again, every statistics should be taken with a grain of salt; there are biases like Hudson's higher frequency of releases and potential difference in how two systems count downloads, but nonetheless I think Hudson is on the right trajectory. This is a much better situation than what Eric found half a year go.
The PDF is available for everyone to download, and I found it an interesting read. I particularly found it interesting that Bamboo and Continuum got no adoption as far as this research was concerned.