I've started using PDO because the statement support is better, in my opinion. I'm using an ActiveRecord-esque data-access layer, and it's much easier to implement dynamically generated statements. MySQLi's parameter binding must be done in a single function/method call, so if you don't know until runtime how many parameters you'd like to bind, you're forced to use call_user_func_array()
(I believe that's the right function name) for selects. And forget about simple dynamic result binding.
Most of all, I like PDO because it's a very reasonable level of abstraction. It's easy to use it in completely abstracted systems where you don't want to write SQL, but it also makes it easy to use a more optimized, pure query type of system, or to mix-and-match the two.