From what I've heard, it's very well-written and not like a fan-fic. (Saying that, I've read some excellent fan-fic, that's turned out to be well-written and better than the source material. [I don't read LotR fanfiction, though.])
I've also got
Mirkwood: A Novel About JRR Tolkien which does seem to be a fan-fic, and I've heard it's not very well-written. But I haven't read it yet, so I can't comment directly (although the summary did strike me as rather fan-ficcy).
The summary says that the author was inspired to write it because his daughters asked where the heroines were in LoTR, and since I have an issue with people constantly complaining that there aren't many/any strong female characters in Tolkien's world (there are plenty of books that don't have strong female characters in them, but they don't get complained about. Also, the women of Middle-earth are very strong, they just don't play a large role in LotR, which quite frankly is realistic, in my opinion, but that's another debate.

) it struck me as being like fan-fiction. How it got published is rather surprising to me (although the Tolkien Estate did at one point want all copies to be pulped).