Here's my take on this. I totally agree with Borism. The only difference I own the Tamron 18-270 and I love it for what it is.
Yes, the 18-55 IS + 55-250 IS will give you a tad better optical performance, however I know from experience that people who go with the 2 Canon lens solution WILL miss precious shots due to not having the right lens on the camera at the time. And a tiny bit worse IQ photo is still better than no photo at all.
I started out with the Canon lens combo on my 40D a year and a half ago. I liked them but changing lenses often was a bit of hassle to me and I occasionally missed shots due to that. In addition, the plastic mounts on the canon lenses were, although solid, still a little concerning. So I did a bunch of research (including Lightrules' tests) and bought the Tamron 18-270 VC. Got a good deal because at that time Bing cashback (Microsoft Live back then) was 25 %, so the lens cost me only $450.00 shipped, new from an authorized dealer.
But the Canon 18-200 is also a very good lens, I just had a better deal on the Tamron and preferred that lens anyway a bit to the Canon.
Anyway, since I had all three lenses at the same time for about a month, I did a bunch of comparison tests myself, which can be found in the Lens Sample Archive under the Tamron 18-270 VC topic. The results were excellent, I'd say my copy of the Tamron is 98% as good as the Canon combo. So I sold my 2 Canon lenses.
Note that I have other lenses too and I'm not against changing lenses. I do change lenses on my cameras. But going on a vacation or the beach etc. the Tamron superzoom does an excellent job and I don't miss my Canon combo. And let's face it, those Canons aren't pro lenses anyway, so if someone wants ultimate IQ then he/she is not going to use the 18-55 IS or 55-250 IS for that.
And I totally disagree with people who believe that if you don't like or want to change lenses on a DSLR then you might as well get a P&S instead. There is just so much more to DSLRs superiority vs P&S than just being able to change lenses: high ISO performance, depth of field, dynamic range, shutter speed, burst speed, manual control etc. So to call a P&S an alternative to a DSLR + superzoom lens is silly IMHO.
Of course the best solution would be having 2-3 different bodies and put top lenses on each for different purposes for best quality. But then again who wants to carry all that stuff to the zoo or vacation etc. Not me. I have 4 lenses and 2 bodies but I usually only take my older body (40D) and my Tamron on vacation. Superzooms are a compromise but you're not sacrificing as much as some people believe (mostly those who don't own one).