The Image Quality of the Pro1 is still hard to beat. The L lens helps a lot of course, and the 28-200mm Zoom range was the best amongst the powershot for long time. The Pro1 is 5 YEARS OLD and that must be considered, of course.
I think it is not cumbersome, and I use it every day. It was packed with all the bells and whistles of its time and then some more because it was built to test the entry level DSLR of these days, in my opinion.
The worst thing about the Pro1 is the combination of a manual focus difficult to use and its poor ability to auto-focus on low light; the S5 IS is better focusing and I think the G9 and G10 focus better than the Pro1 too.
Ah! The noise level at 400 ISO is also very noticeable.
Returning to the image quality, check the g-series challenges, there are many pictures of the Pro1 and of all the G-series cameras.
On this week challenge, a Pro1 picture won the first place, followed by a G10 and another Pro1