I thought _The Scar_ was very-good-to-excellent.  Possibly half a
notch above _Perdido Street Station_, which slumped a bit in spots.

Mieville managed to surprise me at several points.  Finch fooled me
just as well as he did the hapless protagonist; I didn't quite believe
in the grindylow invasion, but I didn't realize how effective his
treachery would be.  And Uther Doul's final turnaround was a trap very
neatly sprung on both protagonist and reader.  Surprising a reader
like this is quite hard, and I was very impressed.

The infodumps were also handled very well -- Doul's lecture on the
Ghosthead, for instance, tells us as much about Doul as about the
mysterious-vanished-conquerors-who'll-be-key-to-the-plot.  Conversely,
the mysterious bits were left nicely mysterious.  We still don't know
the true shape of Bas Lag, nor what the Ghosthead really were, nor how
Doul got his sword and his fighting skills.  Or even what the
delightfully named monsters in the haunted section of town really are.

And, as others have pointed out, there's a lot of underlying structure
that's not obvious at first glance; the Doul-Finch duality, the
multiple riffs on the concepts of love-healing-scarring.

And the set pieces are impressive.  The mosquito-island, the final
scene of destruction, yadda yadda.

There are weak bits.  The protagonist is a bit too much of a passively
manipulated pawn; Doul is just a bit too omnicompetent.  A few things,
like the death of the avanc and the survival of the Brucolac, fell too
neatly into place -- one had the distinct sense of "everything going
back to the way it was originally, world without end amen."  And now
and then one could hear the sound of the author's personal axes
getting ground.  So the whole dispute ended up being about -- horrors
-- trade; the soulless forces of wicked greed destroying individual
lives and honor as they seek to etc. etc. etc.  Also, is the
government of New Crobuzon ever anything but utterly corrupt,
oppressive and treacherous?

And the magic-tech doesn't seem to have been worked out very
rigorously.  If you stare at it a bit, you get the feeling that
Mieville thought, "It would be cool to have lots of balloons --
handwave, handwave, there are balloons."  This is much more forgivable
in fantasy than SF, of course, and the handwaving is fairly adroit.
Still, you don't want to stare at either the technology or the
sociology too hard.  (Floating pirate city?  Really?)

But these are quibbles.  A good read, highly recommended.

Doug M.