I've used both qTranslate and WPML. They seem to be the 2 best multi-lingual plugins for wordpress. However my opinion is that they are both inadequate.
Seems to be slightly better for advanced users. It works by duplicating all the content tables and linking them together by id. Gross and not ideal, but better than what qTranslate does.
Looks really nice at first, but you will have many problems down the line. It works by inserting special html comments into each post like "--- Japanese starts HERE ---" then it parses and replaces the text with proper language.
The bottom line is that Wordpress was never designed to be multi-lingual. So no matter what plugin you use, it will always be inadequate and will always be implemented in a non-ideal hacky way. These plugins basically take over Wordpress and try to force it to do something it was never designed to do.
Either solution might be ok for a personal blog, or something with not much data, but I would not recommend them for any site where the data is valuable and/or the site needs to be flexible or frequently changed/updated. Neither of these play nicely with other plugins. If you install either of them you will find that many features of Wordpress and other plugins often become un-useable. Upgrading Wordpress becomes a burden and you must wait until the plugin makers have updated their plugin to be compatible with the new version. If you decide to use either of these, please proceed with caution as you will be putting your website's fate into the hands of a single plugin maker.
Wordpress developers themselves acknowledge that there is currently no single optimal solution for this problem.
The only real solution is to petition the makers of Wordpress to change the product to be a real multi-lingual solution out of the box.
That being said, my personal recommendation for a multilingual site would be to use the "Each language in its own WordPress installation" method. Now that Wordpress 3.x supports MU installs out of the box, this is not too hard to setup. Maintenance is slightly more cumbersome, but at least your data will be safe and your site can be stable and updated frequently.