Took it last year and i do'nt know if it has changed since then, but it was pretty spot on. the bio/biochem was a bit easier than the AAMC sample test and their own full length exams, but that could have been sicne it was so short. psych and CArs was tough though, bit tougher than the real thing. passages and q got the feel of the aamc, and it was harder than the aamc question pack material.
overall id say it was a very good exam, better than much of the free stuff out there (esp Khan). take it early on, but remember that its such a shrot test, it wont really diagnose your science skills, only your mcat skills like passage reading, thought processes for questions.
i think that was before my time, by the time I took it in summer 2015, it had been overhauled and i didnt see any similarities beyond some topics on a passage or two. I know the revamped their exams again for 2016, have you taken the "new" diagnostic? Free is Free, can't hurt to take.
No way. no FREE 1/2 test will be better quality than the $X full length exams. i'd say they are similar, but more effort clearly went into the Fl exams to make them aamc like, challenging, and fully explained.