Wow, I really, really loved this one. Just wonderful world-building, and a beautiful, gentle story which has quite a lot to say.
I really loved the two alien races. And the ending was incredible.
Anyway, I always feel less comfortable heaping praise on a story than nitpicking, and I don't have a single nitpick this week, so I'll end here.
Aha! At last I have the opportunity to hold a view that differs from eytanz'. I did like this story, and thought it had some lovely prose and captivating imagery, but it's nowhere near a love for me, and I doubt I'd ever listen to it again.
I had trouble (as others have described) coming up with the scenario in which these creatures could have evolved (especially what their evolutionary use for nerve endings and pain was). I also thought Gruen was particularly slow and stupid when he didn't figure out
for decades wtf Western was talking about with the sand. It seemed so obvious and we're supposed to believe this guy is a deep thinker?
The scene at the black whirlpool was the bomb. In general, I loved the setting. Most of my nitpicks are character-based.
For example, while I agree with bolddeceiver about the perplexing nature of these creatures and how they might have come about, I disagree on how well the 'alienness' of the aliens was depicted. Everyone seemed quite human in their motivations, feelings and actions. Maybe there was some time dilation or compression involved, but all the impulses I took from the story were basic human ones which were easy to identify with. I like my aliens a little less anthropomorphized (or a bit more, with no pretense of really alienizing them).
I also wondered about bathroom breaks. Is that just me? There was so much description of the physical body and Gruen's eating/sleeping/movement process. I just wanted to know if he had an alien style colostomy bag or what. Maybe elimination wasn't poetic enough to be included in the story, though pre-chewed food isn't exactly lovely either.