I have both the Tamron and the 18-55mm. There's not much in it between the two except the size and WR.
I used the 18-55mm for over 5 years, and still think it's great. I only bought the Tamron for the WR (I was heading up to Scotland, and plan on some Rainy Tokyo stuff later this year). I kind hoped the constant 2.8 would be amazing too, but in reality, it's only one stop difference at the long end.
Speaking of the long end, past 60mm or so it is a little soft unless stopped down at least to f4. I also have the 70-300 so if I find myself at the long end I just switch to that. Don't get me wrong, it's still perfectly usable at that length, it's just something I noticed during my initial testing.
Build quality is great, and weather sealing has held up with some HEAVY rain without issues. The stabilisation and AF also seems a little better than the 18-55mm. There is no aperture ring, which I thought would be a big issue for me, but having it mapped to the front dial works. Within 5 minutes I didn't even think about it anymore, it's just muscle memory.
Basically, it's slightly better than the 18-55mm in almost every way, but it is MUCH bigger. As someone who takes my camera everywhere, I find the 18-55mm is on it 80% of the time. It's only if I anticipate bad weather that I'll bother to take the Tamron.