I tried Transifex for a bit - just summarizing my immediate experiences with it:

* Easy to set up, had it up and running within 15 minutes
* No online .po editor - it requires a translator to download a .po editing tool, which causes the workflow to be download, modify, upload
* Close integration with SVN/CVS/BZR, but failed to get it to actually commit to a repository - when you upload a translation you commit to the repository.
* OpenID support, which is nice.

It is a bit like a fancy interface to a source code repository, which shows the statistics of how many strings are translated, but doesn't seem to support any collaboration features beyond locking a translation and uploading/downloading translation files. It does not even tell you what strings are represented in one but not the other.

My showstopper problems with it:
* A translator shouldn't be required to download tools to help out - it limits the amount of people we can crowd source from
* It doesn't offer any kind of connection between the translations / .po files, even if there's a lot of things that could be done to make translation an easier experience.
* It would be better to offer an administrative "merge translation" capability, than the current "upload straight to repository" method.

I've put up a test instance of Transifex up on http://trac.tspre.org:8088/ - use login guest, password guest, - it is based on the Mer project, so there's pre-existing Hildon etc strings loaded into it.