The heavier you smoked before you switched, the easier it is to come out ahead on the finance sheet.
If you smoke a pack a month, or if you only smoke every now and then when you go out drinking, then it's very hard to justify financially. The up front costs take a long time to pay off if you're only "saving" $5 a month by switching.
If you smoke a pack a day, it's easier to justify. 2 packs a day, easier yet.
I smoked 3+ packs per day, until 3 weeks ago. My estimates are that I break even in 2 more weeks, which will be 5 weeks after I started.
My break even would have come sooner if I'd actually quit smoking right away. I've been moving over fairly slowly, but it's been going well. Only 4 cigs yesterday, none so far today. Until a few days ago, I'd cut down from 3 ppd to 1 or 1.5 ppd.