Should I get the 17-55mm IS USM lens?

I would go back and look at the shots you take with your 18-135, and see what focal lengths you use most often. I used a 24-105L as my most-used lens on my 20D, 50D, 7D, and now 7DII. After nine years, it broke, and the cost of repair (nearly $400) prompted me to look at other lenses. I find myself at the long end quite a lot, especially for people pictures, so I knew the 17-55 wasn't for me. I ended up getting the 18-135 STM. So far (I've had it three days), I'm very impressed with it. I don't think the 17-55 is optically better, at least not significantly so. But I've also read that the 18-135 STM is optically better than the original version. If you have the original 18-135, perhaps you will notice a difference. The main advantage of the 17-55 that I can see, at least for me, would be the constant F2.8. But, when I want to shoot with a wide aperture, I use my 35 F2 IS (really excellent lens, I sold my 35 F1.4L after getting it), or my 85 F1.8. If you find yourself shooting at the longer end of your 18-135 a lot, and you're happy with the optical quality, I would suggest adding a fast prime. The new 50 F1.8 STM is supposed to be pretty good, and it's very cheap. The 35 F2 IS is really excellent. Not cheap, but a fair bit cheaper than the 17-55. I don't know whether you can get one cheaper than the deal you're currently looking at. I live in the US, and I certainly couldn't get a new 17-55 for anything close to the price of the 35 F2 IS.