I just finished Ill Met In Lankhmar, a compilation of Fritz Leiber's Fafhrd and Gray Mouser stories - roughly the first half chronologically in-setting, I believe. It was certainly an entertaining ride.
The stories have an in-depth, leisurely descriptive style that lapses only during moments of high action - and sometimes not even then. It's a pleasure to read, though in some stories, it results in much of the story feeling like (very enjoyable) set-up with the resolution squeezed in the last couple pages.
An oddity of when they were written is the attitude towards female characters. It's not necessarily wholly negative, but it certainly comes off dated and a little limited - even when the female characters are strong in their own right.
There's a fair amount of headhopping, which is very nicely done because it moves smoothly back and forth between the two characters. That's definitely something you'd have most editors of action-style fantasy looking at you askance for nowadays, though.
There's an extent to which the characters are unsympathetic. I noticed this mostly in the earlier stories, the origins. Fafhrd in particular ranges between being a cad and an idiot and right back again in his first (chronological) story. Abandoning your pregnant girlfriend to chase after an actress ... and then sticking with said actress after she double-crosses you multiple times? Well, then. ;-)
The humor displays itself more in an attitude, most of the time, than in outright "funny bits." There's an underlying sense of the tongue in cheek, but best of all, it's a realistic tongue in cheek - it's laughing at recognizable human foibles (how they might appear in a fantastic setting, anyhow). So it never distances the reader from the story.
In any case, I have no grand conclusion, except I had a lot of fun reading this.