In almost all situations it would be best to save the coin. Assuming every other card in your hand cost 3+, it still allows you to slightly beat the tempo with one of those cards and to play your Engineer or a "3 drop" on turn two.
Other factors to consider, besides your hand, is your and your opponent's class abilities. If you are playing against a Mage or Druid, they could easily use their ability turn 2 to kill off your creature and you have basically exchanged you 1/1 and the coin for another card on turn 1 (making this a horrible play). Even if it was a rogue, getting in 1 damage would not really be worth the cost.
If you are running a Tempo or Rush style deck such that the majority of the cards are 1 to 3 cost and RNGesus gave you all of your 4+ cost cards, I would still save the Novice Engineer for turn 2 and play a 4 cost minion on turn 3.
Generally speaking, it is usually a bad idea to use the coin on turn 1. However, if you have another good "2 drop" in your hand, that would be the only time I would consider using the coin turn 1. For example, if you use the coin to drop a Faerie Dragon turn 1, you could then play out your Novice Engineer on turn 2.
I am stretching REALLY FAR here to come up with any other ideas where it could be an acceptable play. If your opponent had an ability that would not affect the field, such as a priest, it might not be a horrible play (still, depending on your hand there are probably other uses of the coin that are better).