Because I'm intrinsically cheap, and virtually all of the flying I do now-a-days is VFR, look out the window flying, I used WAC charts as they covered more area (kinda like getting two charts for the price of one).
The problem with the WAC charts was the scale..at 16 statue miles/inch they lack a fair amount of detail compared to Sectional's (about 1/2 the detail ..go figure

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The problem with Sectionals was I was always flying just a wee-bit further than the area they covered, so I needed 5 or 6 of them on hand and inevitably the one I needed would be out of date. I decided to try the air chart system. At the time they only offered WAC charts in the "atlas" format, but the convenience of having them cover the entire U.S. outweighed the "scale" issue.
I've used the air chart system for over 15 years now, and find it to be very useful. I like the size & ease of use. Since air chart started offering Sectionals in the "atlas" format I've been using the "Western VFR Sectional Atlas". Unlike some others who have posted, I find the updates to be easy to understand and use.
GPS's are nice, VOR's, ADF's, RNAV, and LORAN's are all useful, but there is something to be said for being able to stick your finger at a point on a map and know you are "there". Probably has something to do with learning to fly back in the 1960's when knowing how to read and use a map was essential; old habits die hard I guess.