I have the small version for my mini dachshund, hoping it would keep her entertained while i'm away at work.
First you have to buy the "everlasting treats" (not cheap, mind you) and insert it into the toy. My dog slipped it out in 1 minute and gobbled it up. The treats are not sized correctly and they dont last long at all! She is not one of those "brilliant" dogs either.
Now, i just fill it with small kibble or frozen wet food. Even so, it's not difficult for my doxie, usually taking her 2-3 minutes at most. In short, don't waste your money. Try the Kong wobbler or other highly rated puzzle toys for a longer lasting experience.