It's more of a health and safety thing, if you to work indoors e.g. your bedroom, some substances could be detrimental to your health not to mention stink the whole house out, in this case water based paints are safer, but in both cases you should make use of an appropriate mask.
I have used AutoAir paint in the past and had success and failure in equal amounts so I was looking for some House Of Kolor paint to try (urethane).
I couldn't find any locally, there was a place in Doncaster but when I phoned them they had already ceased to stock the range.
I found Specialist Paints on ebay did a bit of research and found they were just down the road from where I live, I had a word with the guys and took the plunge.
Like Malky said "Safety First" after all they are solvent based so wear a proper mask, they are decent paints, the "Pearlz" are really nice colours for what I wanted (model cars and monsters), "Sugar Apple" a very nice pearl green.
Like everything though there are bad points, the bottles they come in are easily knocked over (experience) and don't have a "dropper" cap so you have to take the cap off totally to pour the paint out but the bottles are universal and AutoAir caps do fit the bottles.
And being solvent based the clean up after a session is terrible but that said I do like using the paint.