Well, you do get what you pay for.
There are (off the top of my head), three lenses from canon at 50mm.
f/1.8, f/1.4, and f/1.2
the f/1.8 is a good lens for messing around with. It can be sharp if used well (and a good copy). It can be used to great effect for landscape, and, there is at least one person on here who takes photos of robins with it.
The f/1.4 is around £200 more expensive. For those that like to do portraits, it is meant to be a worthwhile get.
The f/1.2 is around £1000 more again. Some reviews suggest that it really isn't worth it. However, if you need f/1.2, then it is worthwhile the spend.
There is an 800mm lens from Canon, this costs around £8k+ boy would I like to find one of those in my Christmas stocking! But to some people, they wouldn't want this lens (I like birdies).
I think that the 28-135 is a good walkabout lens. It covers the ranges I want. I rarely use my 18-55 kit any more, but I am not sure whether that is because I don't like the quality, or the range. I also have the 10-22mm which I use for landscapes, so I think it is a quality thing. The thing about the 10-22mm though, is that it is ef-s, so if you are thinking of going full-frame at some point, it will not work.
I would vote that the 28-135 would probably be a good compliment. It is a good lens (not a great lens, so if I win the lottery, I might upgrade), it was my second lens, then I went to 70-300 for the birdies.
If you want a landscape lens, then the 10-22mm at £400-£600 (used/new) is something to aim for
If you want a good walkabout lens, then the 28-135 is Ok, I believe it is ~£150-£300
If you want a great walkabout lens, then apparently the 24-105L is good. But it is L. If you want numbers to aim for, look at the L lenses on
www.camerapricebuster.co.uk
My understanding is that good lenses don't get to be L lenses,