As someone with asthma and year-round allergies, I'm always looking for something to help decrease my symptoms. I was first given Singulair in 2000, and I've taken it every day since then. At first it was only indicated for the prevention of asthma symptoms, but a few years ago I saw that it was being advertised for seasonal allergies as well.
I noticed an immediate improvement in my shortness of breath symptoms within the first few days of taking Singulair. The few occasions that I've missed a pill I've also noticed that it's harder for me to breathe. This medication generally makes everyday activities a bit easier if you have asthma.
It is not something that you take if you're having an asthma attack: it works on an everyday basis to reduce substances that cause bronchoconstriction. I still have a rescue inhaler that I keep with me at all times just in case, but I haven't really had to use it in a long time, and I attribute that to the effectiveness of Singulair.
The only disadvantage I can see is the cost. Before I had insurance that covered prescription drugs I paid about $90 for each month's supply. It was still worth the cost because it really gives me a lot more freedom from my symptoms. Now I have a different insurance policy and I pay a $50 copay each month. It's still a lot of money but I wouldn't go without it if it was at all possible.
It's very easy to take one tablet a day and be able to climb stairs and go about my other daily activities without worrying about my asthma. It's definitely worth a try if you or someone you love has asthma.