I like the way Magoosh presents their GMAT practice questions. Once you signed in, you can go to the Dashboard which then takes you to Practice or Results section. In the Practice section, you can customise your "practice" session.
The best thing is you can start and stop anytime, unlike some practices which require you to complete the whole set. Once you completed, you can review your questions. The Results dashboard presents a summary of the questions attempted and how you fare vis-a-viz with an "average taker". I am waiting for a reply from Magoosh on the "average user" profile/definition. Nonetheless, this "average user" benchmark allows me to determine how well I do in terms of pacing (speed), accuracy (correct) and the difficult of the question (score).
In my review, I tried one question to get a feel how the dashboard works.
The question was on a Venn diagram, solving the no. of students who took both Spanish and French. It took me 45sec. In the Results dashboard (above), 86% of the takers (who are these takers?) got the correct answer and "they" took an average of 63sec. Great, I could get it right in less than the time taken by an "average taker". As you attempt more questions, each question (with an ID) will display your results and that of an average user. Use this to gauge your performance. I use "Avg Correct" to gauge the difficulty of the question.
Now, comes the best part of Magoosh. The video and/text explanation is found for each question. Just click on the question ID.
Magoosh offers basic 6 months access for free with 80 questions and for US$79, you get all the questions (and they add from time to time)