I recently purchased the Sigma 18-50 f2.8-4.5. I bought this lens instead of purchasing the t2i with a kit lens cause I knew I was gonna end up selling it any way, so instead I opted for this sigma cause its a faster lens and has much better build quality than the Canon 18-55.( I know cause my sister has one) I've had this lens for about a month now and I love it! It has an optical stabilizer (which helps a lot for handheld shots) and one of my favorite things about it is when you zoom or pull focus the length of the lens doesn't change, its all internal which is great.
The best thing about this lens is that I only paid 165$ on ebay, they usually run for about 200$ but I would say its worth every penny.