A victim of its own hype, in my experience Peter's tends to let down first time customers because of its reputation as a "must try" place for those visiting or passing through Calgary. The lineups are also always ridiculously long, but it must be said that the staff there are used to it and keep things moving fast considering this.
The food is alright...it's hard to screw up a burger and fries, and they don't, but there's nothing spectacular there either. The big draw is their milkshakes; they offer a great selection of ridiculously thick and tasty varieties. Grab a spoon, because they are too thick to drink until you let them sit for a bit.
Is it worth the trip? If you aren't in a rush, sure. Does it deserve the legendary reputation it has established? Probably not.