Is a 18-200mm lens really worth it?

Thinking of buying a sigma lens for my nikon d3000 i know that it has an opening of f3.5. And is it really that good of a lens?

It is good enough for lots of folk - depends how discerning you are!
Certainly very versatile lens. The f/3.5 is only at the 18mm end. At the 200mm end it is only f/6.3!

I've been using a Nikon 18-200mmG VR on my D200 for about 6 years now. That's the *only* lens I use. Works for me. My high end is 5.6, btw.

I won't do third-party glass.

I use my Nikkor AF-S 18-200 mm VR when I shoot motocross.

I can stand in one position on the track and shoot three corners, two straight away's and one jump

No.

It's only good if you don't intend to use any other lens. If you don't mind swapping lenses now and then (which is the true way of the dSLR), then get shorter zooms or don't buy zoom lenses at all. Generally, the shorter the zoom, the better the lens optically. That's the reason most demanding photographers use single focal length lenses exclusively.