Initially looking at the zen kernel, I was thinking 'what's the utility'?... Ok, it's tweaked for the desktop, but I didn't expect much of a difference form the official and the Linux kernel, until I really started to configure it.
It's basically a collection of almost all the patches, enhancements that's around and has been provided for the Linux kernel made into a single kernel – Zen.
These are the key features that I really liked (from Linux-2.6.34) -
•.CFS CPU scheduler – Although I've not tried it much, but it boosts to increase the responsiveness of the kernel. It's coded by veteran kernel programmer Con Kolivas.
•.Tweak for the above or CFQ scheduler and DIRTY_RATIO, BACKGROUND_DIRTY_RATIO – You can set the priority of X to a high value if using CFQ or even tweak the CFS scheduler and even DIRTY_RATIO, BACKGROUND_DIRTY_RATIO which makes sense in desktop systems.
•.Compression mode – You are given 4 compression modes of the kernel including LZO which's the fastest for decompression and increases the boot times by a very small amount (install lzop package for you kernel to compile if you're using this compression method).
•.Additional RCU implementations including Preemptable tree-based hierarchical RCU to boost, again, the responsiveness.
•.Support for -march=native for compilation.
•.Timer frequency – You can set timer frequencies till 10,000 just in case you wanna experiment.
•.Tuxonice – I think this is not in the official kernel, it claims to be a more efficient suspend method.
•.Reiser4 support – Although it's future is uncertain, it's good enough for the desktops. Enjoy the fastest FS for small files.
•.Many more – which I did not notice.
Overall, if you're a veteran Linux user, it makes all sense to replace the default kernel with this one – personally I did see the desktop becomes pretty responsive when using this kernel; and no, it's not placebo effect, I could point out which kernel am I using when either one were started randomly (The original Gentoo kernel and the newly configured Zen kernel.).
It will be very nice if Zen also includes the patches for the Linux unified kernel.