The double-gain could be really handy. I suspect that often the correct play will be to buy Engineer and then immediately trade it in for two of X so that both copies are available after your second shuffle, along with however many of X you were able to buy. It could make the difference between having 3-4 Ironmongers in your second shuffle to having 5-6. Even better if there are multiple different engine parts that are $4 or less, since the two choices can be different, unlike many other multi-gainers. The fact that it disappears is also very handy, since Workshop turns into a dead card. I will buy Engineer on a lot of boards where I wouldn't buy Workshop.
That said, it's completely skippable on boards without desirable cheap kingdom cards. I've experimented with using it to gain double-silver on such boards, and that's just not great, even when you need money. The extra cash doesn't make up for the loss of tempo.
It also might be an interesting buy right before a shuffle if you dud that hand. Sometimes a third shuffle starts generating $6 and $7 when what you really want is more copies of the key $3 - $5 cards, so you often have a situation where paying off the tiny bit of extra debt isn't an issue and it could slightly increase the gaining efficiency of that shuffle. That's what I think we have here: Not a spectacular card, but a card which can, in the right circumstances, provide a tiny edge in efficiency.