The Tamron is actually a very solid lens with an amazing range. If the copy is in good shape, it can be an excellent all around travel lens and all-purpose lens. Certainly, image quality is NOT 'miles behind' - a good kit lens with solid optical quality may be slightly better, and VR has some advantages, but the Tamron 18-250 is about as good as one can get in a wide superzoom, and it can be used for almost any type of photography.
I shoot with a Sony-badged version of this lens (made by Tamron) and have had it for 3 years. Though I swap between 7 different lenses depending on needs, I have relied on the 18-250 to perform in nearly every type of photography one could imagine, and find it holds up amazingly well. My one caveat is that I have the advantage of image stabilization in body, so my 18-250 is stabilized...but don't underestimate the lens or write it off based on guesses about its quality...base it on actual users and get an idea if it would work for you.
Sony DSLR-A68 / Sony 18-250mm / Minolta 50mm F1.7 / Tamron 150-600mm / Minolta 300mm F4 APO
Sony A6300 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / FE70-200mm F4 G OSS / FE70-300mm F4.5-5.6 G OSS / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses