In article <4io304$a9f@inet-nntp-gw-1.us.oracle.com>, jtjioe@us.oracle.com says...

>I hear some people mentioned that going from 28mm to 24mm doesn't
>really experience too much wider coverage as suppose to from 24mm to
>20mm. If this is the case, then do you think is it worth to get
>24mm lens if you already have 28mm.

No - I would look for a 20mm if you intend to keep the 28mm.
Neither the 24 or 28 is very wide - the 20mm begins to be wide
enough to shoot building interiors, pictures in cramped locations,
or to really include a whole building exterior when you can't move
back very far, or a significant part of a large-scale landscape.
And, besides, it is just plain more fun to use a 20mm than
a 24mm lens (though you can also get into more
trouble when photographing people).
Hope This Helps