I've been looking into this same thing because a cyclist friend of mine won't stop talking about her Bluetooth helmet. The reality is there are options that work decently, but it really depends on what you're after. If you just want to connect earbuds to listen to music or GPS while you're riding, brands like Livall and Sena have helmets that do that well. The thing with sensors that detect obstacles and warn you is more complicated: some helmets use cameras and radars for that, but the accuracy isn't great yet. A lot of times it alerts you to stuff that's not relevant, so it ends up being more annoying than useful.
What does work better is basic connectivity. If you go that route, look for one with good battery life (at least a couple days on standby), a microphone that doesn't pick up too much wind noise, and something that's easy to clean. These helmets run between 150 and 300 bucks roughly, so it's not money to throw away. I'd recommend trying one out first if you can, because some models have uncomfortable helmets and it's not worth having the latest tech if you can't wear it for more than half an hour.
As for whether it's just marketing hype, yeah, there's some of that. The idea of obstacle sensors sounds incredible, but in practice you need to change how you ride and rely on your own senses anyway. If you're already riding carefully, a basic Bluetooth smart helmet does more for you than those detection systems. Try starting with that before you spend money on the fancier stuff.