Both sentences are correct, the choice of the tense expresses a different aspect of the action.

• Vous avez l’air d’être l’un d’eux, comme je l’ai été moi-même, autrefois.

  Passé composé: focuses on the action/state itself with no reference to the duration. It can mean it did not last long, but not necessarily.¹

• Vous avez l’air d’être l’un d’eux, comme je l’étais moi-même, autrefois.

  Imparfait: focuses on the duration of the action/state and implies it lasted for quite some time. The subject of the predicate, “je“, is given more importance.²

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If a translation into English is any help:
¹ …, as I once was.
² … as I used to be.