Sorry, it is the MkII version and I'm pretty impressed with it as zooms go.
Yes, I have this one and also think it's good, from around f/8 if you want sharp corners, f/11 at the extremes of the focal range. It's very resistant to flare. I think the contrast could be better at 35mm, and it doesn't seem very forgiving of dirty/poor filters. This is my favourite wide angle lens I've ever owned - and I used to have the legendary Olympus 21mm. There's a new f/4 version coming out within weeks which some think will be better, so now is not the time to sell what is still a good lens.
So that makes 2 very good lenses you do have. I would keep those.
The 70-300mm is heavy but even the 70-200mm f/4 IS is longer and less convenient to carry. That's the one I have, because the 70-300 hadn't been released.
I'd be tempted to replace the 24-105 with a 35mm f/1.4 or other fast single focal-length lens. Canon's 50s are all a bit weird and flawed in one way or another, so if that's your thing do read the reviews carefully.