Are these pots really clear?

Yes. The sides are transparent plastic, so you can see roots and mix without lifting the plant out. Expect some algae film if the pot sits in bright light.

Do they have drainage holes?

Yes, drainage holes are part of the design, which matters for orchids that rot in soggy mix. The included saucer catches the runoff.

What size orchid fits a 6-inch pot?

A 6-inch pot suits a medium phalaenopsis or similar orchid with a root ball a bit smaller than the pot. The pot is about 5.5 inches tall.

Can I use them for plants other than orchids?

Yes. The drainage and clear sides work for other epiphytes and for seedlings where you want to watch roots. For heavy soil plants, a heavier pot may be more stable.

Is this a good value?

Four clear plastic pots with saucers cost more than plain nursery pots, and only one store lists this set, so there is nothing to compare it to. You are paying for the clear walls and the saucers.