I bought a 24-70mk1 and it is working perfectly fine now but...

[RayValdez360]

I just read that  some of them  mess up over time. Is that common for a large percentage of them? Do they have a short lifespan of sharp images due to usage. I will sell it and get a MKII if am  expecting to have soft photos one day during a project.

[wickidwombat]

they can be recalibrated if they start going off or soft

[michi]

There are some plastic auto focus guides in the I series which can degrade after time.  Once they do, the auto focus becomes "sloppy".  The plastic guides can be replaced by Canon.  No idea how long it takes for these guides to degrade, but im sure it's quite a lot of AF actuations.

[Mt Spokane Photography]

Everything wears out over time.  There are mechanical parts internal to a lens that wear, but nothing to worry about, just have a clean and check done every 25-50K shots, they will lube it and note any parts that are badly worn, which is unlikely.
What does cause issues is droppage, or even a sharp rap can crack internal parts, or decenter elements.
I've had five of the 24-70mm L lenses, none were worth the price I paid, my 24-105 was far better IQ wise.

[RayValdez360]

I can't find any issues with it on my 5d MkIII. I am coming from using a 7D with  a 17-55.  My only problem is getting used to  f2.8 being like f1.4. Anyway how do you tell an image is soft. Every time I take a pic I  keep zooming in on my LCD.

[canonian]

My copy of the original 24-70mm f/2.8 was great for the first few years but then became soft unless I stopped down to around 5.6 which is unacceptable IMO.

Sent it in to get calibrated 3 times.  First CPS who actually made it worse so I sent it back and it was a "little" better.  About 6 months later it started getting soft again.  I could't stand it so sent it to a very reputable guy in NJ who is an ex-Canon employee and is well known for his excellent work on Canon gear, especially lenses.  I got it back and it was good again but not quite as good as when I first purchased.  I knew it wouldn't last so when the vII was announced, I sold it on ebay.

I hope this long fricken' wait is worth it.

[kaihp]

Roger Cicala of LensRentals have explained why & how the 24-70 go 'soft' in the long end in this article:
http://www.lensrentals.com/blog/2011/10/the-limits-of-variation
I had my 24-70 checked and they replaced 3 bushings. Waiting for the lens get through the line and the workshop: 3 days. Actual work time: less than 1 day.