Hard in every possible way imaginable. The questions are harder. The timing is more brutal and there are some topics on there which are really not on the DAT. I thought it was good prep but there is a lot of bs to wade through before you know what about Achiever you can take home and what you can leave behind.
Personally, I got the 7 test version and did all 7 of them and felt very very prepared. Even though I was getting 18s and 19 AAs on it, got 24 in the real thing though so that may give you an idea of the difference in difficulty level.
From a bang for your buck point of view though, Achiever is the best. Offers the most number of questions and such for the asking price and 90% of the questions are usably hard. Some passages and some science and pat questions are unnecessarily hard (or even unsolvable or ambiguous) but for the most part its good.