That's a good question to ask, and I would respond to it in three ways.
I. TEST THE FRUITS
"
Elizabeth Boott Duveneck, Apple Tree Branches" (1883)
First, consider the matter empirically: that is, test the fruits (Matthew 7:16). We can see throughout the history of the Catholic Church, down to the present day, people who burned with love for Christ and who were deeply devoted to His Mother. That is, we see several cases in which Marian devotion seems to have helped, rather than hindered, a Christian's commitment to Christ. Where do we see cases in the other direction? It's no good citing nominal Catholics who wear rosaries while shamelessly sinning. All too many nominal Christians (Catholics and Protestants alike) wear crosses while dishonoring the Name of Christ. In those cases, the problem isn't that a love of Mary got in the way of growth in Christian sanctity: it's that they don't have a genuine love for Mary "or" Jesus, or they wouldn't mortally sin.
So what we should be looking for is someone who was committed to Christ, but after taking up proper Catholic Marian devotions, lost his faith, or at least, lost his zeal. If such a person doesn't exist, there don't seem to be the bad fruit that we would expect from a bad tree. In other words, by the test laid out in Matthew 7:16, it seems that we can say that legitimate Marian devotion is good, since it produces immense visible good, and no visible evil.
By that same token, test the fruits of the virulently anti-Marian crowd. See how well (or how poorly) their anti-Marian views exhibit the fruits of the Holy Spirit: "love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control" (Galatians 5:22-23).
II. CONTEMPLATE THE ROLE OF MARY
The New Testament depicts Mary as the Ark of the New Covenant, the New Eve, the Temple Gate surrounding the New Temple, Christ, and the builder of that New Temple. What do all of these images have in common? Two things. First, all of them include PURITY: the Ark was too holy to even be touched (2 Samuel 6:6-7), Eve was created without original or actual sin, the Temple builder had to have bloodless hands (1 Chronicles 28:3) and no one could pass through the Temple Gate other than the Lord (Ezekiel 44:2-3).
Second, each of them is REFERENTIAL. The Ark is holy because it is where the Lord would come (Exodus 25:21-22). The same is true for the Temple Gate and its builder, since the Temple was filled with the Glory of the Lord (2 Chronicles 5:11-14). And sinless Eve is drawn from, and points back to, sinless Adam (Genesis 2:22-23). In other words, Mary is pure because Christ is Divine, and it is right that the person that Our Lord was physically connected to for 9 months be sinless... particularly given that sin cannot enter the presence of God in Heaven (Revelation 21:27). This includes both EXTERNAL purity (Mary's perpetual virginity), but more importantly, it includes her INTERNAL purity (her immaculate conception and sinlessness).
All of Mary's life is in relation to her Son. Who among us can say the same?
III. KNOW YOUR ENEMY
Revelation 12 has a fascinating depiction of the nature of Satanic attacks. First, here is Rev. 12:1-6, with a heavenly depiction of the Mother of God:
"
Woman of the Apocalypse",
Hortus deliciarum (1185 A.D.)
AND A GREAT PORTENT APPEARED IN HEAVEN, A WOMAN CLOTHED WITH THE SUN, WITH THE MOON UNDER HER FEET, AND ON HER HEAD A CROWN OF TWELVE STARS; SHE WAS WITH CHILD AND SHE CRIED OUT IN HER PANGS OF BIRTH, IN ANGUISH FOR DELIVERY. AND ANOTHER PORTENT APPEARED IN HEAVEN; BEHOLD, A GREAT RED DRAGON, WITH SEVEN HEADS AND TEN HORNS, AND SEVEN DIADEMS UPON HIS HEADS. HIS TAIL SWEPT DOWN A THIRD OF THE STARS OF HEAVEN, AND CAST THEM TO THE EARTH. AND THE DRAGON STOOD BEFORE THE WOMAN WHO WAS ABOUT TO BEAR A CHILD, THAT HE MIGHT DEVOUR HER CHILD WHEN SHE BROUGHT IT FORTH; SHE BROUGHT FORTH A MALE CHILD, ONE WHO IS TO RULE ALL THE NATIONS WITH A ROD OF IRON, but her child was caught up to God and to his throne, and the woman fled into the wilderness, where she has a place prepared by God, in which to be nourished for one thousand two hundred and sixty days. Her Male Child is Christ, of course. If that wasn't plain enough from context, the reference to Him ruling with an iron scepter is to Psalm 2:9. Reading Psalm 2:7-9 makes it clear that it's referring to the Only Begotten Son, and this Psalm is explicitly applied to Christ in Acts 13:33. The dragon is Satan (Rev. 12:9):
And the great dragon was thrown down, that ancient serpent, who is called the Devil and Satan, the deceiver of the whole world -- he was thrown down to the earth, and his angels were thrown down with him. And here is how Satan reacts to losing (Rev. 12:13-17):
AND WHEN THE DRAGON SAW THAT HE HAD BEEN THROWN DOWN TO THE EARTH, HE PURSUED THE WOMAN WHO HAD BORNE THE MALE CHILD. But the woman was given the two wings of the great eagle that she might fly from the serpent into the wilderness, to the place where she is to be nourished for a time, and times, and half a time. The serpent poured water like a river out of his mouth after the woman, to sweep her away with the flood. But the earth came to the help of the woman, and the earth opened its mouth and swallowed the river which the dragon had poured from his mouth. THEN THE DRAGON WAS ANGRY WITH THE WOMAN, AND WENT OFF TO MAKE WAR ON THE REST OF HER OFFSPRING, ON THOSE WHO KEEP THE COMMANDMENTS OF GOD AND BEAR TESTIMONY TO JESUS. So when the devil realizes that he can't defeat Christ, he attacks His Mother (Rev. 12:13), and out of hatred for "her", persecutes the Church, since all of those who hold to the testimony of Christ are her children (Rev. 12:17). To get to Adam, Satan attacked Eve. To get to Christ, Satan attacks Mary.
CONCLUSION
Understood properly, Jesus and Mary point towards each other, since love is not jealous (1 Cor. 13:4). Mary's last words in Scripture about Christ are emblematic: "Do whatever He tells you" (John 2:5). So are Christ's last words about Mary: "behold, your Mother" (John 19:27). As Revelation 12 shows, it's the devil who tries to separate Christ from His Mother, and His Mother from the followers of Christ.
So while I understand the hesitation that some Protestants (and even some Catholics) have towards Marian devotion, Scripture presents the continuous tradition of Marian devotion as a positive (Luke 1:48), while Satan is depicted as the one seeking to create a division between Mother and Son, and between Mary and the Church.
