If I may chip in some pennies...
Re: open source - You can still make it viable as a commercial product while having it open source. For example, I have a server administration program that's 100% open source but I offer a service to keep it running for them, as well as support (both separately) for different pricing models. This is really the way commercial open source products are going.
Now, on to Blesta and my love fest for it .
I was using Box Billing initially, after going through other solutions (even writing my own which ultimately failed), but their support was horrid. It took 5 days to get a simple cookie-cutter response, so I couldn't justify paying the cheap price for it even if it did work with my control panel at the time.
I went to Client Exec as I used it at my previous employment, but just didn't feel like it fit me (plus the price was too high). So, I embarked on trying something else, and came across Blesta.
To be honest the initial feeling of it wasn't that great, but it grew on me. I was used to the web 2.0 feel of CMSes, but then I saw what Blesta 3 was turning into so I bought into the promo of $99. Been here ever since.
Granted, I feel bad having yet to develop a merchant plugin I said I would develop, but mostly due to moving away from PHP development and getting wrapped up in things. Plus how I have to develop it doesn't make for easy/quick process.