Did anyone put your newborn or infant onto a normal mattress to sleep instead of a baby cot? What do you think of using mattress because it seems more practical that way.

Actually, it depends on indiv preference. There is no right or wrong way as each family's lifestyle differs. We had baby cot but in the end, baby co sleep with us on the bed. When baby was older, we transferred him to mattress because he was old enough to turn and afraid of him falling off the bed. We made sure that the sides are cushioned and all are surrounded by pillows etc.