I had a 36% improvement in a mid-sized Rails 3 app start time using the core load.c patch. Will be trying again on 1.9.3preview1 soon but in theory it should remain similar. DHH also tweeted noting a significant improvement at the time with one of his apps.
It may not be significant to you or in your use case but it seems in some cases there are definitely reasonable gains to be had, which can't be a bad thing.
