you will understand as a Nikon user Canon isn't my speciality
Richard's comment about the 18-135 vs 18-55/55-250 makes sense the more you stretch the lens the more you compromise on quality - its an observation i'd certainly make in the Nikon world. BUT sometimes you need convenience.....
I would definitely buy quality glass, if you buy lots of cheap lenses you'll eventually regret it and end up spending more.
I've used the Tamron 17-50 f2.8 (non VC) and it is excellent. It's advantages over the 18-55 are better build, more versatile - i.e much better in low light more able to blur backgrounds and a decent portrait lens f2.8 at 50mm. Whether it is sharper than the 18-55 when stopped down to f8...... what the 18-55 gives up is build quality.
if it was me I'd consider the Canon 15-85 not as good for low light as the Tamron but you get more telephoto and a hint of wide angle.
However after waffling on for a bit I notice that in the UK the 18-135 is £350 assuming that is your budget for that you could have
the 18-135
http://www.photozone.de/canon-eos/46...n_18135_3556is
jack of all trades master of none won't be better than the 18-55/55-250 combo
the 18-55
http://www.photozone.de/canon-eos/40...855_3556is_50d
and the 55-250
http://www.photozone.de/canon-eos/41...5250_456is_50d
not the last word in build quality but excellent VFM
the 17-50 non vc
http://www.photozone.de/canon-eos/28...report--review
probably the best lens but no image stabilsation.
or the 17-50 VC
http://www.photozone.de/canon-eos/48...750_28vc_canon
the second best lens includes image stabilisation
i wouldn't buy the 18-135 if you want a jack of all trades buy a proper travel lens.
for me its do you want a full set of focal range now ? then get the 18-55/55-250 or a lens that will be the bedrock of your system for years? then get one of the Tamron's