If fabric pots are in direct contact with soil, roots can and will grow through the bottom. As you noted, the air pruning effect only works for the sides.

The other question I have...can some plants have roots too "thick" that end up circling since they are too coarse to penetrate the fabric?

Also - has anyone done or found a comprehensive study comparing root performance in Smart Pots vs. some of the competitors, like Rootmaker, Roottrapper, Air Pots, GeoPots, Root Pots, etc?

I have seen a rootmaker vs smartpot thread on a 420 forum. I can't imagine that there is much difference between their performance.

Smart pots are awsome and yes your roots will grow through the bottom and into the soil. If sitting on dirt, move the pot every day or two if this is an issue. I have seen nice fiberous root balls with 0 circling at the bottom.

I could see some benefits of the roots coming through, such as a bit of extra moisture protection on hot days with drying winds - but you lose the rootpruning. Moving the pots every once in a while would probably break any little roots that come through and mechanically prune them. That's probably what I'll do.

So do thicker rooted plants still get properly rootpruned?

Those "420" growers, whatever your personal opinions of them, do seem to keep up with the newest things and really know their stuff.

One of the great things about fabric containers is that they act like small raised beds if you put them in contact with the soil and don't move them. The earth acts as a giant wick drawing excess moisture out of the pot. At the same time, it keeps the pot from drying out completely. For this reason, you can use a heavier soil mix in a fabric container in contact with the ground. I modify my 5-1-1 mix by replacing the perlite with DE (NAPA FloorDry) and replacing the peat with compost. I don't do that for my regular containers. I strongly recommend NOT moving these pots.