This thing is a life saver. It's louder than most car horns, (which are generally around 100 decibels), and has stopped folks from hitting me countless times on my e-bike.
It's not advertised as such, but if someone thinks they wanna cuss at you while they're driving, you can simply lay on the horn for a few seconds.
It makes elderly folks jump, so you may want a more quiet sound for other messages.
I use other horns for quieter warnings, but this thing WILL be heard inside of cars.
10/10, would buy again.
PSA, this is a toned down version of an oldschool product which holds more air, but this one is more than plenty.
The fact that you can recharge this horn makes it wonderful.

I purchased this item for my amazing 76 year old Grandma who commutes to work and well everywhere on her bicycle. She found that LA drivers were clueless and close to killing her on a few occasions and asked me to find a horn. I needed to find a horn that was 1) easy to use 2) theft resistant 3) easy to maintain and reliable, such as easy battery change, water proof, etc. I asked a couple questions to the buyers here on amazon and got all the answers I need before my purchase.
My Grandma has a 26in townie electra and the tube is long enough to reach the bottle holder BUT the air bottle is small and rattled around and the slight hook on the bottle holder make it fit kinda wonky so my husband used the zip ties to strap it directly under the bottle holder, where it fits like a glove and could easily route the tube with the break tubes already in place. We put the botton on the thumb side of the left grip and its easy to push in a panic or to alert a comatose la driver busy texting. As suggested by a amazon reviewer we also used epoxy on the screw hole to make it more theft proof.
My Grandma has been using it for days now and had no trouble using the air pump at a local gas station to refill it. It is a true air horn so nobody can deny my Granny the right of way. She is safe and extremely happy with the ease of use and reliability. Since there are no batteries we don't have to worry about them corroding in the beach breeze or them going dead when you need them most.
Over all this horn is literally a life saver and I recommend it to ANYBODY who rides in traffic prone cities.
EDIT:
After 3 theft attempts the horn finally was broken. The first and second thief managed to push it around but not remove it or damage it at all. But the 3rd thief was vicious. They managed to rip at it enough to cause it to stop working. Either way, They horn stood up beautifully under a lot of pressure. Without the horrible people trying to steal it I'm sure it would have lasted the test of time.

Cars definitely hear this horn the few times I've used it. It's louder than I was thinking it would be and happy to have it. Only wishes would be that the trigger had better options around placing it as it's one step over from my helmet turn signal buttons, which means I have to lift up to hit it. Maybe I'll flip the two. Right now I have the bottle in my water bottle cage and may re-work that but for now it's on a commute bike and I don't carry water. Price is a little on the higher side but knowing what I know now I'd buy it again.

I wasn't sure whether this horn would be as loud as the reviews suggested; it's always hard to buy things sight unseen (or unheard). But I took a chance, and this thing is amazing. First of all, it came filled with air pressure. Which is great since my pump/gauge hadn't arrived yet. Installation was a piece of cake, two extra zip ties included in the box. Took me a minute to find the shims (pull off the large square velcro patch on the bottle). One phillips screw and that's it. Attached to my handlebars right next to my shifter; I zip-tied the tube from the bottle to my bottle cage in two places to control the cabling and also prevent theft. I can still remove the main lever from the handlebars to fill it, but don't have to worry about it disappearing when I'm in the store. If someone tries to cut the zip ties, they will probably cut the tube, so won't bother. I'm going to be using my bike to replace my car (which I am selling) so I want something that will make me feel safer on the streets and ensure that distracted drivers and the generally-just-clueless do see me. I think the biggest concern here is that you might startle a driver so much he/she might accidentally react in a less than safe manner. That's how loud it is. Atleast as loud as the canned air horns, maybe louder. I haven't had to fill it yet but I don't think it will be a problem since my pump is a shrader fit. If you want to ride with a sense of security, this is the ticket. Just get one. For the price, given the great design and the reusable factor, it's a terrific deal. I took off the cute little bell that came with my bike, which was pretty much useless.
UPDATE 6/13/12: This thing saved my life! I was riding out of Costco's parking lot and some d-bag in a huge SUV failed to look behind him before flooring his car and pulling out quickly. I was just at the edge of his bumper and gave him a blast and he stopped on a dime! If he had kept accelerating, he would have rolled over me and my bike. Worth every single penny spent!!! His windows were up and he was playing some music with a lot of bass, so no way he would have heard me yell or a little bell chime. Also I only need to pump it every couple months; it holds the air really really well. super satisfied!