Recently on his blog Yannick has done some benchmarking comparing mbstring and iconv in PHP 5.2.4 release.

Following up on my previous post about the differences between the mbstring and iconv international characters libraries (which resulted in a tentative conclusion that nobody knew anything about those differences), and particularly the comments by Nicola, we have combined forces (mostly efforts from Nicola, actually) to provide you with a little benchmarking, if that can help you decide.

His code for the test script is included (for you to gather your own results) and a full listing of his results comparing the effects of possible caching, running up to ten executions. You can download the text file that he ran the script on here.