Have you ever read all the details about guts of database engines available for MySQL? Some articles are really surprising. Most are convinced, that MyISAM is good for reads with small percentage of writes. But an article on mysqlperformance blog shows something different. And it is not only special kind of queries (e.g. JOINS) but also really simple SELECTS.
I haven't tested it yet with my application. Its time for change. MyISAM stays on slaves, but masters need fresh look and tuning.