So far I have both Lankhmar and Nehwon. I really need to get a group together to put the rules as applied to the test. With that in mind I have to say that I am very impressed.
It is important to understand that the two books attempt to be faithful to the world of Leiber who never really set out to make a role playing campaign module. Thus there are many elements of Nehwon and Lankhmar that will remain not covered because Leiber never bothered covering them. Unlike the old AD&D scenario they don't make things up from whole cloth when there is an omission in Leiber's works.
The rules seem to work with the gritty world of Nehwon. The modified magic system seems to present an intereting twist to the dark arts while allowing the other magical arts to be vague and somewhat weak compared to the vile cursing throwing wizards.
The adventure in Nehwon is very interesting. A little toward the railroad side, but this is supposed to be Leiber's world and I'm sure that if he had been a DM his two players would have constantly accused him of outright railroading almost all the time!
I highly recomend giving both of them a read. Then I would suggest a test drive in actual play. The rest is up to you!