Some very interesting opinions in this thread...
Hmmmm...
L glass is made from the finest optical glass known to man. L zooms are created with exactly the same glass as L primes. Exactly. The reason that any zoom lens will tend to be less sharp at wide apertures is because they have more glass in them to create the zoom feature. You get an L zoom such as the infamous 70-200 f2.8 IS or the 100-400 and stop it down a couple of stops, you will not be able to discern a zoom lens photo from a prime.
When I made the switch from standard Canon lenses, which are very, very fine lenses, the change was absolutely astounding. The color rendition, contract and sharpness I saw in my photos was shocking. A huge definite bundle of wow-factor came when I made the change to L.
And with the dSLRs it's not really that the images come out inherently soft, but the fact that they come out neutral. If you're coming over from a P&S, most of those cameras add gobs of in camera sharpening.
The reason that dSLRs are set neutral is that a more discerning photographer will more than likely be using the camera and we tend to want our images neutral and with little or no in camera processing whatsoever.
As it is, our images do come out of the camera slightly sharpened. You can kick up some sharpening in the camera but we never suggest it. Forget in camera processing and do your own sharpening.
But I digress, back on topic. If you don't have L glass it's hard to justify the cost. I have been there and never could understand all the hub-bub over this super expensive L glass. Everyone's running around peeing their pants over their L lenses. I thought they were nuts! There was NO WAY I was going to pay MORE for a lens than what I paid for my freaking camera.
Until I took a bite of that L lens pie.
Wow! Amazing. Yes, I do still have a few non-L lenses and probably will have them for awhile, but the next two lenses on my wish list are both L-glass.
Whether you make the plunge or not is a personal decision you have to make. Is it worth it? For me it most certainly is and I'll recommend L-glass any day of the week.
They are expensive, but I do feel, they are worth every single penny I have paid for them and the photos show it.
Good luck with your decision.