Free marketing is very attractive to many people, especially marketers when they are just starting out. Safelists can be a good way to get some free traffic to your website, and Herculist is a prime example of a good-quality safelist.
If you haven't heard of safelists, they are simply email lists which you sign up to and agree to receive advertising from. In return for this, you are allowed to send emails to the list as well. Free members are usually restricted to sending to a small part of the list. Paying to upgrade will get you access to many more emails.
Herculist free members can send an email to 1000 random members every day - which obviously adds up to about 30,000 peer month. Since the membership is well over 70,000, you have a good chance of sending your message to different people every day.
The problem with safelists is that few people actually want to read all the emails they receive, so you will not get many clicks for each email sent out. Herculist will probably get you one or two clicks every time you send to 1000 members. Not a lot, but you have to consider the fact that it is free, and doesn't take very long.
Upgrading in Herculist can enable you to send a message to the entire membership every day - over 70,000 people. So now you are likely to get many more clicks, and probably one or two subscribers on average for most of your mailings. I upgraded recently, and have definitely seen the click rate go up significantly.