I’ll speak for qTranslate, I’ve used it a few times and been pretty happy with it. What it sets out to do – letting you offer translations of post content in multiple languages and switch between them – it does pretty well.
It gets a bit dicey when you try to extend it beyond alternate language versions of posts, though. I had to hack a lot to use the language filters on category titles and descriptions, for instance, or products with different currencies, etc.
For the most part, qtranslate’s approach is pretty straightforward: you put the various languages within shortcodes so the title “My Title” would be entered like this:
[lang_en]My Title[/lang_en][lang_es]Mi Titulo[/lang_es] ...
The plugin hacks up the default post editor nicely to handle that bit transparently to you. But if you want to translate other parts of the site – widget content, etc. – you have to enter that markup yourself. Works well for basic blogs, but if you’re trying to do more with internationalization, you might want to look at some of the other discussions that have happened about offering multi-lingual content. Here’s a few: