It can sometimes be used positively, such as in the expression 'he has a lust for life'. However, generally you should stick with passion. It's okay to be passionate about your work, for example, but if you are lustful for your work, it sounds very weird. When you're talking about the attraction between people, lust is seen as entirely physical, while passion is something more profound and romantic. You can lust after someone you see on the street, but you only feel passion for someone you know fairly well.

Lust means something more like.. just total desire that you are not in control of and can make you do something stupid. But it depends on the context, it might be a good thing if both people are happy about the 'lust'. This was the word used for a long time to describe a sin in the Bible, so that's one reason it is slightly negative. Passion has to do with having really strong feelings, but ones that you can control and won't make you act as stupidly.