I love the idea that we humans could've already made first contact, but it's not common knowledge. This book had an interesting take on that, which is that the humans who meet them may not have called them aliens, or possibly didn't even think of them as aliens at all. Therefore, the record of what happened has been lost in history.
Apart from the idea, I found the pacing to be extremely slow and not to my taste. A few times I nearly put the book down, although more from external forces than the pacing (Brandon Sanderson's Alloy of Law had arrived and I was dying to read it). The characterizations were great, although as I said, I thought the plot was a little slow.