I have been using GMAT club tests for practice. They have been really helpful. I especially love the solutions that are directly available post completing the tests. These solutions give ideas from different members and open your mind to a different style of thinking.
I like the solutions provided by Bunnel. I believe he is on of the gurus on the portal providing regular Math support. I liked the collections that were created by Bunnel with tricky DS questions. I have used the timed mode in GMAT club tests to solve these questions. They help you to understand where you stand as well as in case you falter you have solutions by other members on the forum.
Thank you, guys, for this compilation they have been useful in my GMAT preparation.