As others have noted, the alien cheela, despite being only 0.5 mm tall, have far more depth than the human characters in this novel. This story is about the cheela, and the humans serve mainly as a plot device. Further, the cheela are a *very* different from us, in almost every way -- size, speed, environment, and culture. This will doubtless put many readers off, as there is little to relate to our frame of reference. If you prefer your aliens to be humans with bumpy noses, look elsewhere.
If, on the other hand, the idea of aliens that really *are* alien appeals to you, this is a great read. Reading about the development of civilization in such an alien context is fascinating. There are parts where the story of the cheela becomes very compelling. Swift-Killer's Climb is nothing short of heroic (yes, something unexciting can still be heroic). The brief "Visit" between humans and Cheela is a blast.
I don't give it five stars, because the human characters *are* flat. It may well be impossible to place both truly alien beings and interesting humans in the same story (as opposed to two different, co-located stories). A separate human story would still be something, though, and it appears the author either did not even try to develop one, or failed completely. The humans are a plot device -- they give the author a reason to tell the story of the cheela, nothing more. Likewise, the human discovery of the "Egg" and the expedition reads as very contrived. Every work of fiction is contrived, of course, but good fiction keeps the reader from noticing. These drawbacks mar what would otherwise be a true masterpiece of science fiction.
While it is radical fiction, this book is based on a strong foundation in reality -- both science fact and scientific speculation. The successful combination of strict science and pure fantasy is part of what makes for great "hard sci-fi", and this book has it, in spades. It truly does "stretch the mind", as one cover blurb says.
Strangely, I also found the story to be inspirational. The aliens live their lives in fullness in their frame of reference, but a cheela lifespan is only 30 of our minutes. Reconciling those differences and establishing a conversation over many generations is, to me, a truly noble act, for humans and cheela alike. Despite the differences between them and us, the cheela are people. If I can be as good a person as the cheela in this story, I will have lived my life well.