I'm using ModernBill with Kayako v2 intergrated. I run the billing on the 1st of each mth, and new accounts that come in after the 22nd of the mth, pay the remaining 8 days plus the next mth, and get billed the following month. If a new client signs up before the 22nd of the mth, they pay the remaining days til the end of the mth, and then on the 1st of the next mth, they pay their normal charge.
I own the copy of ModernBill. It was only a few hundred dollars (I think $300.00) and it's better for me owning the software, rather than paying for it by the mth.
Something that I did do a little differently, was the order links go straight to that plan and payment cycle's page. There's 2 steps my order process skips, by linking directly, with that plan and payment cycle's variables in the url. I think this looks neater, and the less steps in the order process the better, imo.
I'm more than satisified with MB. The monthly batches run smooth as silk, and the incoming new accounts work fine. I used to run anniversery billing, but that was back with httpme, and when I was using Paysystems.com as the recurring processing etc. Now I have MB + eWay.com.au Gateway + St George Merchant account.
I probably only use a small portion of what MB can do, but I take from it what I need, and what works best for me.