I found Colour of Magic fine once I got into it, but it took me several attempts before it clicked for me - usually made it perhaps 50 pages in and then failed to pick up to again, so restarted a few years later. Whereas when I started Guards! Guards!, I couldn't put it down and blitzed through it in no time.
I particularly found that the Ankh-Morpork of Guards! Guards! was a far more interesting and settled setting than how it was depicted in Colour of Magic (where I thought it struggled to rise above being a generic fantasy parody and so never really caught my attention).