I don't plan to get into a habit of meta-reviewing, but I have read coalescent and immortalradical here, and ninebelow here, also greengolux's observations, and papersky's praise. I am much more towards the coalescent and papersky end of the spectrum. I really liked it. I thought that it does indeed add something new to the old sf theme of first contact between humans and aliens. It takes the premise of Vernor Vinge's A Deepness in the Sky, a book I really didn't like at all, and does it a whole lot better - basically, the aliens on their planet have a society which feels much more like ours than do the humans in the approaching spaceship. I thought the various cultures and subcultures, both human and alien, were convincingly fleshed out. (Planets in sf novels are too often portrayed as having just one culture and one language - in extreme cases, appearing to possess a single time zone.) MacLeod is on top form in both depth and humour in his portrayal of the intellectual shock of the encounter for both humans and aliens.
I did feel the novel had one glaring weakness, shared with most of the classics of the hard sf genre to which it clearly belongs. We find out very little about the characters' inner lives. Much of the human side of the story is conveyed through the blog of a teenage girl, which is frankly much more reminiscent of the author's own blog than of the real thing at the younger end of livejournal; I guess I must be reading more teenage blogs than Ken does (and I don't read them much at all). The human characters jump in and out of bed with each other and suffer little emotional embarrassment; as for the aliens, this is the one respect in which we really don't get inside their heads.
However, it's going on my Hugo nominations list.