Can I play a reaction spell in response to my opponent playing a unit?

Ruling: No, you cannot react to a unit being played. Permanents resolve immediately without priority passing upon being finalized on the chain.

Nuances:

• If a unit has a "When I'm played" trigger, you can react to that trigger, but you are not reacting to the unit itself
• Additional costs (like effects that must be paid to play the unit) must be paid before the unit can finalize from the chain
• "When I'm played" triggers happen after a unit resolves and always look back at the immediately resolving action