We use Twig. I've worked with Smarty and plain HTML/PHP in ZF but Twig provides a good balance between flexibility and enforcement of SoC. It is somewhat complex, as is Smarty, but our front-enders are very enthusiastic about it. And based on what little work I've done using it, I already like it better than those two alternatives as well.
So on the question whether or not to learn Smarty: don't bother. Some years ago Smarty was a de-facto standard for templating in a professional environment, but unless you expect a lot of exposure to legacy systems I really wouldn't bother.