I assume you mean the Tamron 18-270mm f/3.5-6.3 Di II VC not VX
I have had this as my only lens on my 500D for just over a year and find it excellent. I wanted one lens to travel with and for general good quality shots that offered good stabilization control and this set up was recommended by all the camera shops I went into. This is my first DSLR set up, so I can't compare but I have been very happy with its IQ, the VC is fantastic as I shake a little, it is so good it lets me shoot with lower shutter speeds for low light conditions, hand held even at 270mm. I have used it for action shots of my lurchers, motor racing, outdoor and indoor events, including school plays in poor light, a little bit of macro and video and it has never let me down. Two of my friends have bought this lens after trying mine and they are very happy as well.
This is one image that I hope shows how good the IQ, AF and VC are. f8 ISO 640 1/320 @270mm in JPEG. He caught me by suprise hence the settings and it looks a lot brighter than it was on the day.
The negatives are the zoom is a little stiff at about 100mm but you get used to tilting it up or down to get gravity to help, it is not exactly lightweight but still passable to carry round all day which I do regularly. My biggest gripe though is that the price has come down from £500 to £350 in the UK with the new one coming out. At that price I am tempted to buy another one.