I am always a bit worried about those jacks of all trades. Usually the
designers must have compromised at some point.  An 11x xoom factor is very
large and those lenses will invariably have at least one weak spot
somewhere. They just cannot be good everywhere.

For technical quality you may want to check out slrgear.com. They don't have
a test for the new Sigma yet, but the test for the old lens might give some
indication.

jue

Nikon is hard to beat. I doubt that any comparable Sigma would be better
than a Nikon lens. The 18-200 VR seems to be good. The Sigma has 6.3 versus
5.6 of the Nikon.
I have one Sigma, a 70-300, but it's crap. Very bad. Last time I bought
Sigma.

Replies from people who have not used either lens are worthless.
Welcome to this group.

The 18-200 class is very convenient as an all in one for travelling and
clearly this design entails significant compromises in performance. You
would not want this to be the only lens you own but it may be the only lens
you would want to drag around with you under many circumstances.

I own both the Sigma 18-200 (non-OS) and the Nikon 18-200. Supposedly the
glass in the Sigma 18-200 OS is a different design.
OS or VR are worth the price premium.
Optically, after many comparison shots of the same subject with both lenses
on the same camera, I can assure you there are differences between their
performances. However neither one is the clear winner.
Neither is all that outstanding at the wide end. The Sigma vignettes wide
open at the wide end more than the Nikon, but the Nikon is not free of this.
My Nikon has visibly more barrel distortion at the wide end than my Sigma,
which really surprised me (a photo of a grid does not lie!). Otherwise they
are not very distinguishable, the Nikon not significantly better optically
than the Sigma. They are reasonably sharp, reasonably contrasty and not
plagued by flare compared to older 28-200 designs.
My experience with purchased versions of these lenses, and others, leads me
to conlude that mass market publications like Pop Photo are doing honest
tests of cherry picked lenses. For example PP favorably reviews the Nikon
55-200. I returned two purchased samples as they were among the worst lenses
I have touched, both optically and mechanically.
If you want to save money on the Sigma 18-200 OS you are not losing much, if
anything, compared to the Nikon and will save a few hundred dollars.
Spend what you save on Photoshop: it is not difficult to correct the
distortion or vignetting if it is even apparent to you in a photograph.

In article
<12bea253-2e13-42b8-b3af-218cc1365157@e10g2000prf.googlegroups.com>,

the nikon 18-200vr is a very good lens, considering what it is.  it's
not perfect, but considering its zoom range, it does fairly well.

the sigma 18-200os is cheaper, but it's not *that* much cheaper, and
not only is the optical quality not as good as the nikon, but the
stabilization is less effective.  you can even *see* the difference in
the stabilization between the two lenses, it's that obvious.  plus, the
sigma is f/6.3 at 200mm which is beyond what the autofocus system
officially supports, although it will generally work most of the time.

If you already have the "kit" 18 - 55mm lens with your D40, the ideal
second lens to buy is the 55 - 200mm VR, as it's a compact and lightweight
lens with excellent performance.  It's a Nikon lens, not a Sigma.  You
will probably get better optical performance than the 18 - 200mm lens as
well, and still have a lightweight and easily portable combination.

Of the two lenses you mention, I would save the extra money for the Nikon.
Of the 3rd-party lenses, I hear there is also a Tamron 18 - 250mm which
may be worth a look.

David