This lens is an f/5.0 at 150mm and f/6.5 from 200mm to 500mm. That means it's a pretty slow lens and you WILL have a bit of trouble freezing action in low light situations.
The only way around is bump your ISO up, what produces more noise.
The thing is, you can't really have an lens that is long, light AND cheap. You have to sacrifice one, and I think for the price it's incredibly hard to beat. For instance, the Canon 100-400L is f/5.6 at 400mm (faster), and lighter.. but it's almost double the price.
The Canon 500mm f/4L has the same focal length, but weights 3 times more and costs 4 times more. Something gotta give.
Now, you have an D90 and its sensor is one of the very best at the moment, so you shouldn't be afraid of bumping your ISO to 1600 or even 3200 to get faster shutter speeds. If you have an flash gun, that becomes even better.
On daylight (even overcast days) it will do the job pretty well. Under sunlight you can freeze action for almost any situation you can imagine.
For shooting right before and right after sunrise/sunset, the ISO will do the trick. Anything other than that and you'll have trouble without a flash gun. I use mine with an 580EXII and an Better Beamer for those situations and is the only way I could find to get good photos with it under those conditions.
Now, on the other hand you gotta know this lens has an 4 stop OS (image stabilizer, or VR for you Nikonians
) that allows for shake-free photos with shutter speed up to 1/50
That's just AMAZING for a lens of this class.
So.. bottom line:
If you want to freeze action under low light (take pictures without any motion blur from your subject), this may not be the lens for you (but I seriously doubt you'll find anything closer at that price range).
If you're worried about camera shake, then just buy it. This baby can hold up very very sharp photos up to 1/50 and will amaze you.