I've had an ATC since day one with my 22.5 WSM. The overnight smokes are a breeze. Most ATC's are adaptive, so if your charcoal is burning a little hotter or cooler, they adjust the amount of airflow accordingly.
I just recently got the Stoker (upgraded from an Auber ATC) it really is spectacular when you can monitor your WSM and even change temps from a computer in your house (or, if set up properly, anywhere via the Internet.)
As mentioned earlier, Stokerlog is awesome...you can see a graph of the pit temp, food temp, and when the fan was on or off.
The big question - is it worth it? IMO the Stoker is probably overkill for the average backyard cook. It has some really cool features, and makes your temps very stable. However, as many have pointed out, the temps really only have to be within a reasonable range to be effective. Many, many, MANY WSM owners find the smoker very easy to regulate and maintain temps without a lot of fiddling with vents. Also, there are other brands that are far cheaper than the $400+ the Stoker will set you back, so if you don't want all the bells and whistles but you want your temps regulated, you could go that way as well (my Auber was around $140 if I recall.)
I'm somewhat of a techie at heart, so I love having a "dashboard" and the graphs and the constant feedback of the smoke. A group of friends and I also plan to enter a few friendly BBQ contests this summer, so I went ahead and popped for the Stoker. To each his own...