Question: I have a Canon 550D which I've been using for around a year now. I really love it and have learned a great deal with this and the 18 - 135mm kit lens. But now with experience I feel the need to get some better glass. I won't be buying a new body any time soon because I figure investing in some good glass is more important. 

As I have looked around, I see quite a few good used L lenses I can get in ebay for good bargain prices. But I know that the EF lenses don't have the same focal length in the cropped sensors. Is it a wise decision to buy these EF lenses or buy the cropped sensor alternatives?

I have the following lenses in mind that I am looking to buy down the line from the canon range over the next several months.

16-35mm f/2.8L, 24-70mm f/2.8L, 70-200 f/4L.

Are these apt choices for cropped sensors? I know most pros with FFs use these but I am a bit skeptic that they will serve the same purpose for me. If so what are the alternatives for this that have almost as good image quality?

Answer: 
I bought a Canon 24-70mm f/2.8 L lens for my Canon 450D crop sensor body before I had purchased the full-frame Canon 5DMKII. By going this route, I had time to learn my lens on a crop body well before I could afford the full frame of the 5D. I even borrowed and used 70-200mm f/2.8 L lenses on the 450D body, which gave an additional crop factor for "perceived" zoom with little else to complain about. 

Positives

* Owning a great lens at a lower price-point to buying a full-frame body
* Shooting great images without waiting around

Negatives

* Only knowing that I wasn't getting the "best" out of my lens in terms of the crop factor preventing me from achieving a "true" 24mm.

Note

* This topic does come up a lot, but the implication is that the intention exists to move to full frame. Plenty of photographers are happy with crop models, but these questions are usually handled in a "should I get the lens before the full frame body" sense. If so, go for it!

The difference between your kit lens and the 24-70 will astound you (with clarity). It will also delight you (when you realize how much sharper your shots are). And then it will depress you (when you remember how much these lens cost).
Short form: 

In general it is wise then, and only then, when you seriously consider moving to a full frame body later. 

If you don't then it is just a case-by-case decistion, as yours obviously is. You are thinking of a number of specific lens for now. 
Then, when the choice is just beween an EF lens and an EF-S consider the following: 

* There is noting wrong with using an EF lens on a crop body. Nor is there anything wrong with a lens park made of both kinds.
* A crop body would use the best parts of the projected image only. Important when you take reviews into account. You can then ignore their less than optimal quality in the edges, that some of them may show.
* EF-S lenses of the same optical quality when used on crop bodies can be significantly cheaper than the EF lense that you compare it with.