ASF is good. So is ABC Financial, who I use. They're both essentially the same. I outsource stuff like this. Don't reinvent the wheel. If you want to grow, you don't want to be chasing money all the time. Huge drain on time and energy. Get someone else to do it. It's worth the 4-5% that you will pay.
ASF and ABC are both very feature-rich. They don't just bill. You can use them for "club enhancement" (internal marketing to your existing members); you can use them for check in and member activity tracking; profit center management (register sales, inventory). You can effectively manage your entire operation through this software (or multiple clubs).
I use a supplemental service to ABC called "Retention Management" which allows me to do email blasts to my members. It also tracks their usage. If a member doesn't show up for a couple of weeks it sends them an email (which I wrote initially) that checks up on them, talk to them about the positive virtues of sticking with it (etc), and tells them we'd like to see them again. At 4 weeks and 6 weeks it automatically sends out more emails to the customer, eventually offering them free refresher orientations or training sessions to help get the customer back in the door, or even offering a free smoothie.
That program has been particularly helpful to my business. Keeping your customers active and connected to the gym builds a more solid receivable by decreasing attrition rates. It is available without ABC, but it is about twice as expensive. I don't know if they work with ASF, but I would assume that they probably do.
ABC Financial Services - Club Billing Services & EFT - Club Software & Online Services - Club Marketing & Custom Design - Service & Support for the Health and Fitness Industry