They're great. I've once had to use one in Ethiopia after I lost my water purification tabs. They had tons of these in a nearby village, given to them by organizations, and kids would use them to drink from puddles.
Well, I've tried it, and indeed, water really is fine with it. Now the "straw" part may imply that water flows freely and thus filtration isn't great. Well, not really, it's still a filter, you have to suck really hard to get water running through it. It's actually just a filter. If you took your average bottle filter and sucked through it, you'd have the same thing. I didn't get sick, and just so you know, Malaria was widespread there.
I myself needed to fill my bottles, so I sacrificed my hydratation pack to fill it with muddy water, and then had the water pass through the tube and into the straw, hence using it as a regular filter, and letting gravity and water pressure do the trick. It wasn't exactly practical, but it worked.
I've since bought a bunch of these, and always take one with me as part of my first aid kit. They're durable enough to be used for hundreds of liters, and even then it depends on how dirty that water really is.
Their only downside is their impracticality to be used as a regular filter, hence why they're my secondary mean to filter water, because it makes filtering water for storage near impossible. Which means, you can only take what you can drink, and thus have to follow rivers.
Now their strong point also is that you can filter water without requiring a container, which beats water purification tabs. pumps, UV cleaning, boiling...
I actually went camping once with nothing but a straw; made my pack so much lighter, not having to carry any water nor any mean to boil it. Of course I planned the thing, I knew there were enough lakes and rivers in our path. And it was a greately pleasing experience.
Pic related, that's the ones I use.
