higher nicotine levels will have more throat hit, it shouldn't be harsh to an ex-smoker unless you're unlucky enough to be one of the few people with a sensitivity to one of the ingredients.
some people detect a slight change in flavours with different nicotine levels, I never did. it's not a huge difference, anyway, some people say the low or zero nicotine level is the more pure flavour, like i said, I never noticed a difference.
some people need 24mg, some people can't handle 24 and start on 6 or 12. there's no way of finding oot withoot testing what's right for you.