The first book of the series aged quite a lot and feels fairly mediocre nowadays. The second book was a greater improvement which I called a classic of sword and sorcery. This book sadly ran out of steam after the first story. It is called The Cloud of Hate and is as good as any in the second book - I suspect it was an inertia which kept it so.
After the great start the pace of the plot came to a screeching halt. The duo started to behave out of characters which were established earlier. There was one plot hole big enough to drive a couple of naval aircraft carriers side by side through: is The Great Mouser a magic user, or not? In his origin story he is an apprentice to a wizard and does show some magic abilities. In the second book these abilities are conveniently forgotten and he is a typical swordsman, now the abilities (again conveniently) came back. So what is the deal???
Do not get me wrong, the prose is really good, but the plot promptly goes nowhere in the middle part. The story When the Sea-King's Away contains beautiful descriptions of undersea side, but if you stop reading and think a little you realize that nothing actually happened during the whole length of it.
There is also the longest novella of the collection called Adept's Gambit. While it contains some interesting ideas, it still feels stale. It does not help any that both Fafhrd and The Gray Mouser act like complete morons all the way until the end. I really mean it; they play passive roles in the drama only reacting to the events and never actually acting to prevent them.
A good writing quality and really mediocre stories most of the time yield 3 star rating for the series. At this point I am really not sure whether the rest of the series is worth reading.
This review is a copy/paste of my BookLikes one: http://gene.booklikes.com/post/982219/after-the-first-story-it-goes-downhill-fast