A bit out dated, and not sure how much of it has changed, but probably still worth a look for many of you.
http://www.developmentseed.org/blog/internationalization/comparison_chart
The 1st 2 comments really captured my attention here:
Take i18n module for Drupal 5 for example. Translating a node/term means creating a new one with the new language and associating the translated nodes/terms. This contradicts to the semantics of localization (i.e., there is a single entity which has multilingual properties) and therefore makes many modules not cooperate with the i18n system (e.g. Taxonomy Defaults).
the drupal 6 version of i18n will support both approaches: localizable terms and truly multilingual taxonomies.
There's already a beta version.
I've hired a developer and we're in the middle of developer a multi-lingual site in D5, and one of the problems I've encountered is exactly that! Translations create separate nodes and they don't share the comments! I don't mind the URLs being different, as I think that can be corrected w/ pathauto:
en/node/1 --> en/beautiful
ja/node/2 --> ja/beautiful (OR) ja/きれい