Can I use namespaces in my plugin?

You can totally use namespaces in your project/theme/plugin!

The only drawback is that you introduce a PHP 5.3 dependency, while WordPress itself only requires 5.2.4.

I guess in 2015 this is not a big issue, even when distributing a plugin to a wide community (opensource or envato).

Roots team is using namespaces in their themes, plugins, etc: https://github.com/roots/sage