I only have the TTMIK Level 1 workbook and use it with their free PDF's and audio recordings (not sure about their grammar books). I would recommend that you take advantage of their free resources first before buying the workbook. It's a bit sparse compared to what you would find in, say, traditional school textbooks like the Sogang Korean or Ehwa.

Also if you live in a city that has an online e-book library, you can probably borrow some TTMIK e-books online through Overdrive. I was able to request a few of them at my own library.

Recently I switched over to this book and it covers a lot more than TTMIK. Be sure to read the reviews and click the "look inside" link on Amazon to see if you'll like it.