I've used them in the past. I didn't punch holes in them but placed my six-pack cells inside them. They worked well IMO. After my seeds sprouted I took the tops off and put the whole pan under lights until it was time to pot up.

I've successfully used them like flats. Another good "tray" is a kitty litter box. I think I paid $2 for them and they last several years.

George
Tahlequah, OK

I too have used them successfully for both indoor and outdoor sowing.

Outdoor - punch drain holes. I either fill with potting mix and plant seed or use as a holder for the cups that I winter sow in.

Indoor - no drainage holes and use them as a flat to hold indoor sowen containers.

Sandy