I personally like to write my own. Suits me much better. Plus, when you need something just a certain way, you can code it in yourself, and not have to find work-arounds from others' code.
I've fiddled with Smarty, but didn't like it. Never heard of flexy though, but usually using PEAR is pretty nice & easy (considering it's written by the same people that write PHP). Pretty much, it's up to you and what your needs are. If you don't need the full blown Smarty or Flexy system, write your own. If you can't seem to get what you want out of it, write your own. But the ultimate decision is up to you.
~Brett