I'm a big model enthusiast and visited several such displays throughout Europe.
Until now, I regarded the Hamburg one being the best, but now, honestly the St. Pet's doesn't have a match - in one word it's amazing.
I see many people here focus this as a primary kids attraction which I can only partially agree. Kids observe this display as a toy, for me it's a piece of art.
The display is split into two huge sections (west and east Russia) in one enormous room, divided by the central passageway (marks the Europe-Asia border).
Every Russia major city with its landmarks, as well all regions are geographically displayed with millions of tiny objects. Hundreds of moving trains, cars, planes, working plants, mines, oil platforms, ski and sea resorts,....you name it, it's there. Every 15 min the room turns dark and display illuminated showing the night scene which is again amazing.
Unseen attention to details with high technology makes you stare at this in disbelief. Make sure to read the plaques showing the number of something and many interesting facts.
Grand Maket is not in the prime tourist area, but in a residential part of town with nothing else to see around, but worth every effort coming.