I don't know that anyone is saying the IQ of the 24-105mm on APS-C is not good, it's just that there are other lenses that are slightly better. I've tested both the 24-105mm f/4L IS and the 16-35mm f/2.8L II on my 7D, against the EF-S 17-55mm f/2.8 IS, and while the first two are very good, the 17-55mm is slightly better (in addition to being a more generally useful focal length than either of the L lenses since one lacks a wide AoV and the other lacks a telephoto AoV).
I have a 7D and several lenses. My favorite all-around lens is the 24-105 f/4 L IS USM. On a crop frame camera it works great for me. Very sharp, weather sealed and IS to boost. Not a fast lens so you'll need something else for low light and action, but for me this lens stays on my 7D most of the time.
I mostly use primes (L and Zeiss) on a 5D MkII, my only zoom is the 24-105 f4. For me is a workhorse, very useful and reasonable image quality. But, if you are going to use it on the beach, you may expect a lot of flare. It's the major drawback of this lens.
24-105 is my first L lens. It gives good pictures(not tack sharp pics) when shot outdoor, but when shot indoor and when light is of the essence it fails miserably.It does a slimy job when compared with the pics taken with primes or lenses with big apertures. I have the canon primes 24mm,100mm and 135mm and 50mm(sigma) and when I compare the pics taken with 24-105, this lens does not even come close with the pics I took with primes. If you have a good camera like 5D series, I would strongly recommend you to get primes or lenses with big apertures.
I have the 10-22 3.5-4.5 Canon, 17-50mm 2.8 Tamron, 24-105 L Canon, and 70-200 2.8 L Canon lenses (also a 30mm 1.4 Sigma and 120-400 Sigma) and if I'm doing a lot of walking around in a decently lit area and don't want to change lenses it's the 24-105mm I use. I don't see myself buying the 24-70mm 2.8 L Canon as good as I am sure it is. If I didn't already have the 10-22 and the 24-105 I would. Obviously I don't really need any filling in between those and my next purchases with either by the 50 or 85 1.2 L's or a 100-400 L Canon.
Finacee has the 17-55 2.8 EF-S canon, 100mm 2.8 Canon. Both those lenses are impressive for not being L glass. I think my Tamron is a better value if you aren't a professional as it is smaller, lighter and around 35% of the cost of the canon 17-55 EF-S. I've never actively done side-by-side comparisons of the 17-55 and 24-105 Canons... I'll have to give it a shot some day.