I bought the Tamron 17-50mm f2.8 XR Di II lens on Friday and while it isn't L glass (what is?) it is the first non-consumer lens in my bag.
All of the things you would hope were there, sharp, fast, fast focus, good color, etc.. What I didn't expect is how bright it is. I knew the wider apeture would pass more light but the viewfinder seems brighter then it is with my 50mm 1.8! The higher quality optics seem to allow more light to pass then my other lenses. I didn't expect that!
Just to be sure I did a test. I set my tripod up in the house and got my Canon 28-90 f4-5.6 out. I set the camera to apeture priority and set it at 5.6. I set both lenses to 35mm and half pressed the shutter button. With the Tamron the shutter speed was 1/125 and the Canon was 1/100.
I've had L glass on my wish list and have been near to buying on several occassions but other priorities have changed that plan each time. I'll be saving much harder now!
I'll post a review of the Tamron when it is listed in the reviews section. It's not perfect but overall I am very pleased with it.