[WARNING]
Just a warning for those using or thinking of using R4 on recent (>=2.6.12 kernels)
The current R4 (1.0.5) is a patch for mm kernels where the effort is to include it eventually.
It includes some changes (additional plugins) that make it INCOMPATIBLE with earlier partitions based on 1.0.4
SIMPLY mounting the partition seems to modify it.
Once an older partition is mounted with a 1.0.5 capable kernel it will no longer mount with and earlier kernel: ie your previous stable kernel and many R4 aware live CDs for example.
There is a way back but it's a bit of hassle
http://forums.gentoo.org/viewtopic.php?p=3019057
Basically you need to make a choice between <2.6.12 with 1.0.4 or the later kernels and stick with one or the other.
Knowing this in advance helps in making the right choice and taking suitable precautions.
Backwards compatability has never been high on the R4 feature list but this should have been clearly signalled.
Maybe this is one area where improvement could be enforced as a condition of kernel inclusion.
[/WARNING]
I agree with earlier posts , the atomic write is the key feature for me.
I got in a mess recently when a careless error filled my root partition with several linux ISO images. The result was that the system could not even handle 'ls' with crapping itself. I had to hit the reset button. Thanks to atomic writes I was able to boot to a CD delete one file and I was back in business without any corruption.
I have been in power-off situations with ext3 in the past that have ended a lot less joyfully.
I'm not saying ext3 is crap because of that, it's a case of tools for the job. I've never had to waste day untangling the mess after a change in the kernel support for ext3 either. That's why there's a delay in getting R4 into the kernel and why it will be great once it gets accepted. This sort of thing will not happen.
It seems that Herr Raiser has created a very impressive fs with minimal outside help. If kernel inclusion can oblige some more stability and the addition of some other minds with a wider outlook I think this will become a major force in Linux in the not too distant future.
I continue to use R4 on several partitions, as always with due care and attention.

Linux, because I'd rather own a free OS than steal one that's not worth paying for.
Gentoo because I'm a masochist
AthlonXP-M on A7N8X. Portage ~x86