I've done some searches on the web for polymer gel crystals - you
know, those crystals that you can either buy separately or by as a
"moisture control" or "smart soil" premix, that are supposed to absorb
100-1000 times their weight in water and make more water and nutrients
available to roots.

I can't find ANYTHING negative on the web about them. There are links
to tons of studies where they say that adding the crystals is good for
plants, some say it's so good it should be ground up, mixed with water
into a slurry, and your plant's roots should be dipped in the stuff!

And of course, the labels on the bags all tout how great the crystals
are.

However, I did find something interesting in the book, "Ortho's All
About Container Gardening," available at most Lowe's / Home Depot type
stores (or Amazon). On page 27, the following is written:

"Polymer Gel: Pros and Cons

Water-holding polymer gel has received much attention, but its use is
controversial. In isolation, polymer gel crystals can absorb a lot of
moisture, expanding up to 100 times their weight in distilled water.
In soil, however, ions decrease the water-holding capacity of polymer
gel by as much as 85%. Research shows that at the low rates
recommended on product labels and offered in some potting mixes,
polymer gel is ineffective in increasing the water available to roots.
Organic matter is less expensive, so it can be used in potting mix in
much greater quantities than polymer gel to hold far more available
water. To be effective, polymer gel must be mixed into potting soil at
such high volumes that its expense and excessive shrinkage preclude
its use."

In searching the web, I can't find anything to corroborate these
statements.

I'd be interested in hearing your opinions.