Big switch is always painful. So does it when you are switching from relatively simple and clean qTranslate to the monstrosity of WPML.

Why am I doing that? Users deserve faster loading times for their webpages. qTranslate is OK when you are handling 2 languages with not much content in every post or page. It is certainly not OK, when you have many custom loops and lots of content on every page. Load time can increase twice, or even thrice if you have 3 languages setup.

WPML saves you from that pain, but it adds its own. The main one – you have to ensure all of your other plugins are WPML-compatible.