My primary usage is personal, though I also use it for business. I have several trunks. I have a "private" personal line, as does my my wife. I also have a "commercial" personal line that I use for businesses that require a "home phone number" and I think might have a legitimate reason to call me. I also have a "junk" line that I use for internet purchases, businesses that should never have a legitimate reason to contact me, etc. It goes direct to voicemail.

I also have a line I use for "business" -- it's the primary line I use at my office (our office PBX forwards my calls to it so that it functions like an extension). That way I can work at home when I want to, but to my clients, opposing counsel and courts (I'm a lawyer), I'm "in the office." I also get calls to my home phones in the office, which is very convenient, particularly when I'm calling the cable company or the bank and it wants to recognize me by my phone number.

As I noted in another post, I have an Obi device that connects my cellphone via bluetooth to my system so that, when I'm home, calls to my cellphone ring and can be answered on any phone in my house, which is very convenient. I ever have a cheap wireless phone in my bathroom so I can hear the phone if I'm in the shower (or otherwise engaged).

I can send and receive faxes through my PBX from any of my computers anywhere I happen to be, and use that for both business and personal needs.

Finally, my system lets me make and receive calls on any of my phone numbers from anywhere I am in the world (as long as I have internet access).

It all depends on what you want from your phone system. I really like the convenience of having a phone system that works exactly the way I want and can be configured to do whatever I want it to do. Until relatively recently, this wasn't possible and, particularly, not for free.