I used Topscore only for practice tests, and as far as science goes, it was pretty close. I'd say it was a bit more difficult than the real DAT, but not by much. My science scores were a bit higher on the real DAT than they were on Topscore. As far as everything else, I thought Topscore was easier than the real DAT. The reading comp on topscore isn't bad, but it is a tad bit easier than the real DAT, if only because it is so cut and dry.
Topscore's biggest problem, in my opinion, is QR. They need more question variety, and they need to make it a bit more difficult to really simulate the DAT. At least, I felt that way after my DAT. It's tough to make many generalizations with a test like this.