I have. They aren't bad. I generally have an awesome night's sleep and I am/was a tad claustrophobic.

Tips: Build on high ground (otherwise your pit will become a tub)
-Use synthetic material to shed water from the roof (plastic sheet)
-If/when you encounter roots dig them way back and cut them off (so the sap doesn't pool into your pit)
-If you are long term pile a large amount of debris on top of the pit as typically they compress a fair deal (the debris on top that is). Otherwise they end up looking like shallow graves.
-Use a shovel not a digging stick and make sure the boards on the top are thick enough not to sag when someone walks across them.
-Buy/make a tiny periscope or dental mirror. The pits are soundproof and exciting one while others are present seriously defeats the purpose of all the sneakiness in the first place.
-If you are going to leave them for sometime ensure that you remove all the bedding from them. Mice will be every so happy if you forget to do this. "Honey, you're not going to believe this but someone made a massive underground cavern for us! And they left fresh bedding!"

Glocker36, you going to the Pine Barrens?