What's the catch with 'free deposit' bonuses? Need advice!

I've been looking at a few new online casinos and they all advertise these huge 'free deposit' bonuses, which sounds amazing. However, I'm really confused about the fine print. Are these bonuses genuinely valuable, or are the wagering requirements so high that it's impossible to withdraw any winnings? Could someone explain how these promotions actually work in practice? I want to make sure I understand the full terms before I commit any money. Any advice from experienced players would be greatly appreciated.