The more focused your list... the better your results will be.
As for your question, no two people will answer it the same I imagine. Some people experience great results, others dont - which has to do with everything from their style to their market (and a thousand things in between).
Some things you can do:
- Be very clear on your site about what your list offers
- Make the sign-up process SIMPLE
- Be sure to deliver what you promised (#1) in every issue
- Request feedback, do surveys/polls... keep it interactive if possible
- Offer GOOD content (what YOU would love to get if YOU were a subscriber)