Some reviews say yes. But since it costs as much as an EF equivalent, why not name it as 'L'? Is there any better lenses in this range (EF or EF-S)?
I used this lens as my main walkaround lens and as a workhorse for event shooting for about 2 years. The image quality was excellent and definitely on par with the most of the L lenses I've used.
Where this lens isn't "L" is in the build quality. Whereas most L lenses are primarily metal and weather-resistant, the EF-S 17-55/2.8 has a lot of plastic and feels like it's not quite as sturdy as L lenses such as the 24-70.
A couple other minor pieces of note: the lens does not include the hood in the package; it's a separate purchase (and a spendy one at that). Also, it's an EF-S lens, and thus far only the EF lenses have been designated as L.
Canon has never designated an EF-S lens 'L', perhaps for marketing reasons. But a more practical difference is that the 17-55mm lens is not weather-sealed (a must for L lenses). I believe in general the build quality of the 17-55mm is not up to par with the L lenses.
The 17-55mm lens is very well known for its image quality, which surpasses many L lenses, according to reviews. This is probably what people mean when they say it is an L grade lens.
An L-series lens also has one (or several) UD elements - see http://www.canon.com/camera-museum/tech/room/hotaru.html. This 17-55 2.8IS does infact have a UD element which many reviewers and the like are using to claim this as an L-grade lens, except it's the wrong mount, being EF-S and not possible to use on Full-format models.
NO its not is the simple answer.
Just because it performs better than many other lenses optically doesn't make it better, following that logic you could ask 'Is the 50mm f1.4 really an L lens?', there is a bit more to it than that. There is another post here that asks the question of 'what makes an L lens an L lens' and links to this article by canon.