I moved up from a 18-70 + 55-200VR combo to a 18-200VR. Never regret the decision at all!
Yes, i lose the superior sharpness and quick af of the 18-70 but i gain versatility which is the selling point. The 55-200VR is no better than the 55-200 range of the 18-200VR...serious...
Friends have poked fun and chided me for using a "lazy man's lens" while they use their f2.8s and L lenses but I'm able to get most of the shots most of the time cause i simply DO NOT NEED TO CHANGE LENS!!! On top of that, I get VR throughout the whole focal range.
Edit: Seriously there's no diff to the IQ of the 18-55 + 55-200vr combo VS the 18-200vr in REAL WORLD USAGE. If you simply need the best IQ, get the f2.8 zooms.
Yes. Once you've gone 2.8, even if it is a cheapo Tamron one, you will never want to go back.