Just finished it myself over the holiday weekend. ~22 hours or so. Best $40 I've spent on a game in a while. Went in w/ super low expectations and judged Starlord prematurely based on the trailers. Was blown away by this. The environments are what I thought were really unique. They were all vastly different and actually felt alien. Amazing pacing as well. It kept you in each chapter just long enough.
Like many of you have stated, I'd agree that the combat was the weakest part. However, once you gain all 4 abilities for each guardian, it actually felt pretty good. I think if there were a few more and the cooldowns were reduced, while increasing MOB HP, it would be perfect. I found myself waiting often times, and if the CD's had maybe 5-10seconds off their timer, or a way to reduce them I would have zero complaints on that front. Highly recommend, even at full price (although no reason to w/ Steam holiday sales).