Can an opponent play a reaction card (like Stupefy) in response to a 'when you play me' triggered ability on a unit (like Faithful Manufactor)?

Ruling: Yes, triggered abilities use the chain and each player gets an opportunity to react before the ability resolves, starting with the controller of the next ability.

Sequence:

• Unit with 'when you play me' ability is played
• Triggered ability goes on the chain
• Players get opportunity to respond (starting with the controller of the next ability)
• If no responses, the triggered ability resolves

Nuances:

• Opponents cannot react after the triggered ability resolves, so they cannot target units or effects created by that ability