Hi, I tried the CertGear SCJP mock exam version. It costs around $28 USD for a 180 day trial period. The full version costs around $48 and has 300 questions. I downloaded it's free evaluation version which had only 20 questions with full fledged explanations. My experiencs : Not worth it.
But why do yo want to go for that when there are a number of free mock exams available on the net ? I feel these small mocks also give valid and sufficient explanations and practice tests for clearing the SCJP exam.
There are even a few free downloadable mock JAR files on the net which you can simply install to your Java CLASSPATH and enjoy the same functionality as any other paid version. Maybe even better !!
Apart from that, you may purchase the Khalid Mughal book which comes with a CD having a 2 full mock test engines...1 from Khalid himself and the other from WhizLabs. Both with good enough explanations and things to play around with.