The 18-55mm IS can be a good lens. The one I got with my T1i (500D) was very sharp, it just needed a fair bit of light. Others have had worse impression of it, so perhaps I just got lucky. You're already used to the variable aperture from your existing lens. If you're fine with the 18-55mm's variable aperture and resulting shutter speeds, then yes, grab one of the newer ones. They vastly improved the optics when they added IS. The even newer ones may have even more improvements.

The Tamron 17-50 VC was noticeably softer than the non-VC one, which is why people only recommend the non-VC.