GMAT Strategy

I am considering taking a GMAT course to better structure my studying. Due to frequent travelling, my only choice is online.

I researched online, and here are a few companies who offer online courses compatible with my time zone:
KAPLAN UK
Veritas
Manhattan GMAT
Manhattan Review
Knewton

If any of you have taken an online course, can you please share your experience, and what online course you would recommend?

I am already familiar with many of the math concept, so I need a course that's a little advanced and go over tough problems, not just basic ones.

Would appreciate your input on this matter.

Thank you in advance.