Greyhawk doesn't mention an attack roll for the arrow. Later versions of the spell went either way; Holmes I believe was the first to mention an attack roll, while AD&D turns the spell into an automatic hit weapon. There's an argument that requiring an attack roll punishes 1st level magic users who take this as their one and only spell; cast the spell, roll badly, and your spell is a waste. Of course, the same thing could happen if you take Charm Person as your one and only spell, but at least your odds are better. I suggest the spell should be auto hit, but a clever reaction from the victim allows a saving throw for half damage; this keeps it only a tiny bit better than Fireball or Lightning Bolt.
I'd make one other change, though: when the caster's level is high enough to create multiple missiles, I think I would restrict them to one attack per round, rather than having a machine-gun effect. This is to help distinguish it from those third-level ranged attacks already mentioned; instead of doing all its damage at once, it doles it out in small bits.