Dat Achiever is worth the money...but helps to keep this stuff in mind...
Pros
Great study tool
Excellent for preparing you for the test format, timing, and procedure (the setup is almost identical)
Gives you an idea of your strengths and weaknesses
Is much more difficult than the DAT, making the actual DAT seem easy by comparison
Cons
Questions are very very difficult
Makes you want to kill yourself
Seeing the scores after taking the practice tests was unbelievably demoralizing, and the closest I have felt in my life to having a nervous breakdown
All that said...well worth the money, it makes you study harder, and by using the questions as indicators of the topics to review, you can really get yourself prepared for the dat.
The day before I took the dat, I took the last practice test and got (off the top of my head) ~ 15, 17, 19, 18, 16, 18. And the next day on the dat got 20, 21, 19, 21, 21, 25. That said, doing badly on the Achiever tests doesn't mean you will do well, but don't get down if your scores are low, it will make you study harder, and the DAT will seem easier by comparison.
Also, for what its worth, I considered Ochem to be my strongest subject, being my highest score (at 19) consistantly on the practice tests, however it was my lowest score (at 19 again) on the dat.