MySQL 5.1 introduces row based replication, a way of replicating data that fixes many inconsistencies of the statement based replication, the standard method used by MySQL so far.
The good: row based replication solves some problems when replicating the result of non deterministic functions, such as UUID() or NOW().
The bad: row-based replication may break existing applications, where you count on the quirks of statement based replication to execute conditionally (updates base on @@server_id, for example), and may perform badly on updates applied to very large tables.
Currently, MySQL 5.1.28 has the mixed mode enabled by default. When in this mode, binary logging switches to row-based when there is a risk of breaking replication. Except that there are some cases left out, like Bug#39701 (load_file()).
Mixed mode is supposed to be the best of two worlds. You can run fast updates on large tables and not risking inconsistent results on non-deterministic functions. The recent addition to mysqlbinlog allows an easy decoding of the ugly looking row-based statements.
Would you switch to mixed mode or stay in statement mode? There are requests of reverting the default to statement based binlog format, to maintain compatibility with old applications.
Personally, I would go for the mixed mode. But it depends on your applications and your personal experience. What's your take?