I didn't switch...
Because I often have to shoot in dusty conditions, I was looking for a better sealed lens than a 28-135 I'd been using and begged/borrowed to try both 24-105 and 24-70. I ended up buying a 24-70.
What surprised me was how close image quality was with all three of these lenses. Of course, the two L-series are a couple mm wider and better built... but the 28-135 ain't bad, it's typical "USM/gold stripe" build.
Each lens has it's nuances. The 28-135 is a little soft at 135mm, but surprisingly good at other focal lengths. Of course, it's also only f5.6 at 135mm. The 24-105 has pretty strong vignetting at 24mm... though it will mostly only show up if using the camera on full frame cameras. The IQ of the 24-70 is darned good throughout and has the best potential for strong background blurs... it's just big, heavy and expensive, with a massive lens hood (that makes sense when you see how the lens zooms).
None of them are truly "sealed", but the L's are definitely better sealed than the 28-135 and feel like they will survive longer. On the other hand, I can buy three or four 28-135s off my local Craigslist, for what a 24-105 will set me back.
All three have USM that makes for fast, accurate focusing. All three are EF lenses, full frame designs, so fully usable on either crop or full frame. Some don't like the range of these on crop, prefer a wider "walkaround" zoom... Personally I like it and don't mind complementing it with a wider zoom (I use a Tokina 12-24 as my only "crop specific" lens, but other such as the Canon 10-22 would serve just as well).
All three are pretty darned close focusing. The 24-70 is closest of the bunch, 24-105 a little less closes, and the 28-135 has the least magnification potential (without adding extension rings).
The 28-135 is prone to "zoom creep". To a lesser degree, it shows up sometimes with the 24-105. Haven't ever noticed it myself or heard others mentioin it much with the 24-70.
I don't mind having IS on a midrange zoom... but it's not a high priority for me on these shorter focal lengths, so I wasn't worried at all that the 24-70 doesn't have it. As camera ISO performance has improved, it's made IS less critical... Though I still really appreciate it on longer lenses. It's just not a very high priority feature for me on shorter focal lengths, though of course I'll take it if it's included and adds little or no cost.
The 24-70/2.8 ended up in my camera bag. I'm not going to be quick to upgrade to the Mark II (let the price settle a bit! or maybe I can win the lottery).
And I ended up buying another copy of the 28-135 to have as backup/loaner lens.
I agree that a combo of a slower zoom along with one or two or three faster primes can be a good setup too.