The premium exam provided is very similar to the actual exams. I've tried the practice exam here for a little more than 40 times. Scored an average of 80% for the practices. I scored 89% for the actual SAP Certification. Note than SAP SD exam requires a 65% to pass, hence you have to aim a little higher than 65% in order to successfully clear the exam. The exam questions are slightly trickier than the practice exam but very very close to the practice questions.
In my opinion, this premium exam is very useful. However, you also need to spend time to study through your notes too. Read through the notes, and around 3 weeks before the actual exam, start practicing the full 80-questions exams twice a day, everyday. Note down the mistakes and learn from it and make sure you do not make the same mistakes in your practice again. A consistent 80% and above in the practicing exam should guarantee you a pass in the actual exam.
All the best.