Despite the middling score I'm giving this, Garden is far from average. I've read a fair share of manga, but have yet to see anything else quite like it, and the atypicalness of the thing makes it difficult to shove into a tidy 10pt scale.
As for the actual plot: Some characters break into a garden and explore. Granted, it's not a normal garden: instead of trees and various shrubbery there are huge, inorganic structures. But that's it. There's no drama, no mystery, no conflict beyond "how do I cross this river made of balls?" or "why is there a mountain made of glass?". Though the story is barebones (to say the least), the bizarre things that the characters encounter usually provide adequate entertainment. It's clear that the mangaka had a lot of fun coming up with ridiculous buildings and landscapes to fill this book with, and absolutely nothing that pops up is expected.
And due to the inorganic content, and the fact that the volume isn't split into chapters, it's like reading the stream-of-consciousness of an autistic architect.
The illustrations mirror the sparse narrative. Screentones, shading, and anything more than the bare minimum of detailing is almost non-existant, and the clean geometric linework gives the whole thing a sterile feel. The character designs were interesting and extremely varied, perhaps to make up for the characters' lack of personality.
Calling anything a 'character' in Garden is a bit of an overstatement. 'Hive mind' would be more appropriate. There are oodles of humanoid… things that wander around occasionally offering opinions, but oftener explanations of their surroundings. Their numbers fluctuate anywhere from five to thousands depending on the situation, and once a character has been shown, they may or may not show up ever again.
The end effect is akin to watching someone else play a videogame. Not one of those story-fueled, action-filled rpgs, though- more something like Katamari Damacy (with a smidge less lunacy). There's no reason to watch/read, you have absolutely no stake in what transpires, and yet it's strangely mesmerizing.