My husband LOVES Zestos.  Anytime we have visitors in town, he wants to take them there.  I'm not quite as convinced, however.  The hamburgers are okay and there are interesting choices (like the Hawaiian burger and the garlicky Godfather) but I definitely don't think that they are the best burgers ever.  The milkshakes are pretty great, though and the "old dinner" feel of the place is nice.
                        
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