My ds6 has asthma. Didn't have to use the vaporizer a lot last winter (maybe not this one either), but it used to be a life saver. And, they recommend the cool mist one now- they say it helps them more than the hot one- but you have to clean it more too. Yep, I did the same as you the first year I used it- didn't clean it often and mold- really helping the asthma huh? Well, my pediatrician said to change the water every day and use white vinegar daily to clean it. He said to use just a hair of bleach and water every once in a while to really clean it good - I did it about once a week.
You are fine with the one you have- just really be sure to clean it good. Use bleach and water- just make sure you rinse it out over and over with plain water when you're done to get the bleach out. Also, if you haven't alreay, look at the part that spins around on the top. You can take that long nozzle off and clean that part too- it is usually really gross.
I also close ds's door during the day while it is running and until I go to bed at night- really helps get the air in his room moist.
Do they really help? That is just a matter of personal opinion. Some say yes, others no. I think it has helped ds over the years and when he is sick- I will continue to use it. With his asthma and as often as he gets bronchitis, I use everything I can and I really do think the vaporizer has helped him..