Is this one of the most under rated canon lenses? Been looking on POTN and dpreview and this lens get very good reviews. Quite a few even say it's IQ is very close to the 24-105L

With the 28-105 only being £160 and the 24-105L £550ish I find that hard to believe but after seeing some snaps from the 28-105 at 5.6 it looks very nice
Reason I ask is I use my Sigma 17-70 for a walkabout lens and am very happy with it's IQ. However, for me personally I'm finding that 70 isn't long enough for me as a walkabout lens and I like the idea of having 105mm on my walkabout lens. The only thing putting me off is 24 or 28 might not be wide enough but you can't have everything.
I haven't decided but I was considering selling the 17-70 and going for the Canon 24-105L (In a few months) if I could afford it but now I've seen the Canon EF 28-105mm f3.5-4.5 USM II it's got me thinking should I get that and also keep the Siggy?