While the Reiser4 file-system has been in-development for the better part of the past decade, it still hasn't been merged into the mainline Linux kernel. Reiser4 is still out-of-tree, doesn't see much new development activity by its limited developers, and the file-system remains tarnished due to its founder, Hans Reiser, being a convicted murderer. However, Edward Shishkin the former Namesys employee, does continue to drive its development forward. Reiser4 was recently updated to work with the more modern Linux 3.5 kernel.
Back in May when I was communicating with Shishkin, he mentioned he was focusing on Reiser4 stability issues than porting the file-system to more modern Linux 3.x kernels compared to the Linux 2.6.39 kernel where it was last supported. However, last month the Reiser4 patch ended up getting released for Linux 3.5.
With Btrfs stabilizing and EXT4 continuing to do well, not many see much potential out of the Reiser4 file-system. It's a very different outlook than when it looked like it might go for mainline inclusion in 2010. In 2011 the story then became that there were only organizational -- not technical -- obstacles for merging Reiser4 to the mainline kernel. In late 2011, Shishkin changed the story and told me that pushing Reiser4 wasn't just of high priority. This past January is when he then told me that merging upstream has mostly just marketing/political aspects and he's not interested in jumping into the mess at this point.
With the Reiser4 file-system patch for the Linux 3.5.3 kernel being released on 8 September, this patch was applied against a vanilla 3.5.3 kernel and then benchmarked against EXT4, Btrfs, XFS, and the earlier ReiserFS. The port to the Linux 3.5 kernel was done by Ivan Shapovalov while Marcin Baczynski did some porting work from 2.6.39 to 3.2. Edward Shishkin continues to be involved on bug-fixing. The Reiser4-for-3.5.3 patch weighs in at 78,975 lines of changes to the Linux kernel.
As for the popularity of the modern Reiser4 file-system, the SourceForge area of the Reiser4 file-system with last month's Linux 3.5.3 kernel patch is reporting just 23 downloads since 8 September.
The EXT4, Btrfs, XFS, ReiserFS, and Reiser4 file-systems were benchmarked with their stock mount options from the Linux 3.5.3 kernel. The disk drive being used for all testing was a high-end 160GB Intel SSD.