From ECJ press release: According to the Advocate General, the dispute is genuine, the question is not merely academic, nor premature or superfluous, but has obvious practical importance and is essential in order to resolve the dispute.
AG says “the power to interpret Article 50... definitively & uniformly is that of the Court of Justice which must carry out considerable interpretative work in order to determine whether or not [A50] allows the notification of the intention to withdraw to be revoked unilaterally.
ECJ statement: A50 allows the unilateral revocation of the Brexit notification, until such time as the withdrawal agreement is formally concluded, provided that the revocation has been decided upon in accordance with UK’s constitutional requirements
...is formally notified to the European Council and “does not involve an abusive practice.”
