The 30" look absolutely nutty coming from a 24". 75% more pixels, and 56% more screen area.
This will give you a good idea:
http://tvcalculator.com/
Select source image at the top to "none".
Put in a 24" screen on the left, 16:10 ratio, 1920x1200 resolution.
Put in a 30" screen on the right, 16:10 ratio, 2560x1600 resolution.
The smaller one is the 24"

. The first time I saw a 30", with such a ridiculously large desktop area I almost shit myself.
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I would definitely say that once you have a 24" screen though, it's diminishing returns. 30" is fantastic, but not necessary for me yet. Though, personally, I couldn't use anything less than 24" and 1920x1200 for my main rig anymore. 1920x1080 is a bit of a stretch. Once you go big, you can't go back!
I recently upgraded from a Dell 2408WFP to a Dell 2709W. Same pixel count, but 26.5% larger screen area. It's comfy!