They are so hideous, and actually make my eyes smart, as I discovered when house hunting.
The only time (apart from obvious causes that were easy to remedy, like an un-emptied nappy bin, or cooking smells) that I've noticed a gross smell in my house was when the spare room wasn't getting hoovered regularly enough, so we just stepped up the cleaning.
All the adverts for them, please can't people just air their homes more??