Im very new to squirrel hunting, but I picked one of these up for 10 bucks and I have had decent success with it.
It's one of the multi calls with the squeal, bark, and chatter.
I've found it depends on what time of the season you are using it and which calls you use.
The bark is usually good for trying to locate them. If you can get one to bark back at you it narrows it down tremendously.
In the late summer / early fall the quarter trick works out quite well as they seem to want to investigate if they hear another squirrel cutting nearby. Also, the squeal works well at this time as well since some of the juveniles are still running around and that maternal instinct will sometimes work in your favor as they nearly run straight to you hoping to help the juvenile. Using the excited chatter, quickly followed by the squeal and kicking nearby leaves or shaking a branch really gets their attention as it mimics the warning followed by a predator attacking a juvenile.
Later on in the season is seems like it isn't nearly as effective as they no longer care about the other squirrels unless they hear one cutting and compete for the food. They mainly keep to themselves rather than chasing each other around or trying to locate one another.
Very late in the season, the chatter will get them worked up since its nearly mating season for them again. You can nearly call one right up to you if using the right camo and if you stay still.
I use a camo bandana to cover my face as they seem to be able to pick out the human face a mile away.
These are just my recent observations. I've called a handful in to me within 15 feet while still hunting, most others, probably the older veterans will just look but really won't come looking for you.
Just my 2 cents. Seems like they work if used properly and at the right times. Right now, in mid-late december, the things basically worthless, best to use your spare change and stalk them as you see them hopping from tree to tree.