i think you should save and wait till you can get a good piece of glass in my own opinion!
Well, I might be getting a good deal on a used one, where I could sell the 28-135 and not really be out much, if anything at all. So money isnt the issue. Just a matter of if its good enough IQ to trade the 28-135 for, so I can have the extra range. I currently use the 18-55 and 28-135 50/50. Not one more then the other, so having it all in 1 would save having to constantly swap lenses to get the wider angle.
Hoping to get a 10-20 soon as well, but that is a special use lens. Probably wouldn't replace my 18-55 with it.