You have to be somewhat careful trying to compare lens between canon and the other vendors. All vendors make crap and good lens so you get difference of opinion based on what is being compared.
You get what you pay for is very true for lens. The glass in a good lens is very very expensive and makes up most the cost difference no matter who makes it.
As you suspect a all in one lens does have its draw backs. Mostly the issue is that these lenses cannot pass as much light and may limit you ability to be able to get photos. This is mostly related to not being able to reduce the shutter speed enough to avoid using a tripod or get blur free photos of moving objects. These lenses also tend to focus slower again because of the amount of light allow.
The most common second lens people buy is a 70-200. All depends on how often you shoot at different distances and if carrying a second lens is a burden.
Still no matter the brand you buy make sure you buy he best lens you can possibly afford. A good lens today will still be a good lens tomorrow. This is why used professional lenses cost almost as much as new ones.
Has any one any experience of, or can offer any advice about, the Tamron 18-270mm lens?
I have a Canon 500D and am very happy with everything so far but feel that maybe the kit lens (18-55mm) has some limitations so I am looking to purchase another lens. The Tamron I have mentioned above seems to be a good all round option but I am worried that a lens to cover all purposes might have a new set of problems, i.e. Jack of all trades but master of none.
Also I have been given contradictory information by 2 different shops, one says the Tamron lens is better in all aspects than the Canon while the other shop has told me the colour definition will not be so good on the Tamron?
Any and all advice on the subject will be greatfully reeived as I don't have an endless budget so don't want to make an expensive mistake.