In October of 2009 I picked up a Sigma 18-250 f/6.3 for my 7D for walk-arounds, so I could shoot any scene without having to switch lenses. My choice was between Sigma 18-250 f/6.3 and Tamron 18-270 f/6.3. The latter didn't have an ultrasonic motor at the time and wasn't focusing as fast as I wanted, so I bought the Sigma 18-250 for about $800 CAD.
I knew that super-zooms won't deliver same quality as lenses with a more moderate zoom range, so it didn't bother me that I had to correct chromatic aberrations (CA) for most shots, that the maximum aperture was very small on the long end or that wide-open shots came out very soft. The lens focused fast and delivered good quality of images for its class.
I shoot almost daily and amount about 50K-60K shots a year. At this rate, I started to see first signs of trouble in about a month, when the inner tube of the lens started to drop down when the lens was tilted down, sometimes as little as 30 degrees, despite that DPReview described that this lens had well-controlled zoom creep in their lens review.
Soon the hood stopped clicking when attached and after few months I noticed that the rubber zoom ring got bloated and quite loose to the point I could spin it around the lens. The ridge of the raised rubber ring also started to block the zoom lock switch. However, the lens still was working fine, so I wasn't worried too much.
In August of 2010 the diaphragm blades started to stick in the closed position once in a while and I took the lens to Sigma for repair. After about six weeks Sigma told me that they can't repair the lens and gave me a new one, but did not extend the original one-year warranty on the lens. It seems that they knew what they were doing.
In November of 2011, I was shooting near a lake when in the middle of a high-speed burst I heard some labored whirring inside the lens and the aperture got stuck in the closed position again, but this time I saw a message on the screen saying that the camera cannot communicate with the lens. I turned the camera off and then on and everything went back to normal, but in a few shots the lens malfunctioned again.
Later that day I cleaned up the contacts and tried the lens again and the aperture got stuck after about a dozen shots. I then tried the EF 28-105 lens and it worked fine for about 30 shots. It was clear at this point that the Sigma 18-250 was gone.
I cleaned up the lens with a cloth dampened in lens cleaning solution and took this shot. Interestingly enough, the cloth turned black after this procedure and the ring appeared to be a bit sticky for a short while, as if some of it got dissolved by the solution. It was also impossible to clean between those narrow grooves without using a toothpick for each groove, which makes the zoom ring a great indicator of how well-used this lens is.
The bottom line is that the Sigma 18-250 is a nice lens, but it has a very limited life span of about 50-60K shots. If you take less than a couple of thousand shots a year, it will last you a life time. If you shoot thousands of pictures a year and it starts looking like the image above, you may start thinking of a replacement lens. Finally, I wouldn't think of buying a used Sigma 18-250 unless it's dirt-cheap and its zoom ring is in pristine condition.