I have the Quantaray, which is a re-badged Tamron. The lens is OK, it works best in bright sunlight, once the light starts to drop off you see quite a loss in quality. It suffers pretty badly from purple fringing.
It's OK for the money, but I wouldn't buy it again, I'd actually buy a Nikon 55-200 for not much more money, or shop for a used Nikon 70-300.
It will be longer than your 18-55mm, but in most/all other respects, it will be fairly similar...so if your 18-55 "won't cut it", this one won't either (unless you are strictly talking about focal length).