I don't know how many of them actually hate it anymore.  But many have throughout history and I am sure some still do.  Those that hate it do so because they believe it is the sort of vain repetition of prayers that our Lord forbade in the gospel and because they think it offers idolatrous worship to Mary.

Protestants hate it primarily because they view it was Mary worship, and considering they don't know how to properly interpret the Bible, they view that verse above as one that states we shouldn't pray the Rosary. Iuvenalis is right, Protestants hate pretty much all Catholics things. You'll come across some Protestants who are open to Catholicism every now and then, but a majority of them are anti-Catholic. Especially the Baptists. Nearly all of the members in my family who are Protestant are Baptist, and all but maybe one or two of them are anti-Catholic.

I think that the real reason why so many Protestants hate the Rosary can be summed up by these reasons:

1) Bad will: This one is obvious, but it needs to be said. Many Protestants are bad willed and will think that Catholics worship Mary based on the Rosary alone. Even if you tell them we don't, they'll still say we're somehow worshiping Mary without knowing it. They don't want to accept anything about Catholicism because Catholicism is too difficult.

2) Stupidity: Some people are just inexplicably unintelligent and the very idea that prayer without Latria worship could be possible is beyond the scope of their very limited imaginations.

3) Anti-Catholic Upbringing: Many people are brought up to hate the Catholic Church and not even understand why. They hate crucifixes mainly because we use it. They hate the sign of the cross because it's Catholic, even though there's really nothing wrong with it. They hate the phrase "full of grace" because it provides a testimony to the immaculacy of the Blessed Mother. They hate the Apostles Creed because we say it. If they do say the Apostles Creed, they edit it in all kinds of ways to avoid making it sound like they agree with us and so on. They just hate anything and everything Catholic.

4) Impatience: Most people are impatient, and Protestants are no exception to this. The Rosary is something that takes time to do, and most do not want to take the time to go through it.

5) Misunderstandings: Many Protestants misunderstand various aspects of the Rosary and so they come to hate it because they're afraid of it. They don't understand why we pray to Mary, so they shun it (but they're so willing to accept Buddhist/Hindu prayer form, which I think is rather strange). This isn't the same as stupidity, which is self-imposed in 99% of cases pertaining to this matter. This is about being mistaken about the true nature of the Rosary. If more Protestants would take the time to understand the history behind the Rosary, they certainly would not shun it.

Generally yes, they do.

I believe that anyone who hates the Rosary,no matter their denomination is simply giving in to the temptation of satan -- any hatred of Our Lady or her legitimate devotions, especially the Rosary and the Brown Scapular, is definitely diabolical. I say this sadly, as members of my own family hate the rosary. Satan is very much afoot in the world today, with so many tabernacles emptied ofOur Lord's true presence, so few real Masses being said. Whenever we see this sort of hatred, remember that it is a sign of the effectiveness of devotion to Mary. When faced with hatred, counter with loving prayer all the more. let hate be the goad to renewed devotion.
Think often of the vision Sister Lucia had of Our Lord and Our Blessed Mother, where Our Lord sadly remarked that so few were willing to pluck the thorns from His Mother's heart. It is up to us to remove those thorns, by prayer and such limited penances as we are able to offer.

Couldn't agree more.

They're heretics.

That's the best reason so far.