Beside ther de-wp there are at least two other projects that use the checked version. One is ru-wp. I didn't see any reports of the practice there, would be interesting too.
We are still running it in a trial mode, we started in August and at some point later the results will be evaluated, and also the inclusion of the proofed version will be discussed, right now the proofed version is not used.
By default the last version is shown, not the checked version, but one can turn on the protection, so that the checked version is shown. I believe we have only two pages protected right now, one of them is the main page. We check articles in the main space, categories, and templates.
Sysops have a patroller flag by default (meaning they can mark any version of any article as checked), unless they explicitly refuse, in addition, we have a couple of hundred patrollers who have been voted in a typically easy vote. Together with 70+ sysops, this must give about 300 patrollers, I did not check the exact number. Also, about a 100 editors have an autopatrol flag (not voted unless self-nominated), meaning if they edit a checked version the article remains checked. Bots also have this flag.
In my opinion, everything is running more or less smootly, even though some strong opinions have been expressed in the beginning, resulting in an arbitrtation case. Unless some major flaws are found, I believe the flagged revisions system will be confirmed in the course of reevaluation (note this is my private opinion).
Cheers Yaroslav