Advantages

• The main advantage is cheap or free long distance to North America and parts of Europe.
• Sometimes you can get rid of your land lines, and save on that too.
• The people who make best use of this are snowbirds or people who travel a lot. Your phone number follows you no matter where you roam. You can use your existing number or get a number with any area code you please. This can save you on cell phone roaming charges, or being charged for a line to a residence you are not currently using.
• Though you must get your Internet access from a local company, you can get your VOIP phone service from any company. Further you can plug into the Internet anywhere in the world, and your number follows you. You just take your combined router/VOIP box with you, or your VOIP software in your laptop.
• VOIP phones often come bundled with features such as call-waiting, caller-id and electronic answering machine.

Gotchas

• You have to buy a gateway box. It is effectively a small computer that does analog to digital conversion and relays the packets out over the Internet. This additional startup cost will eat into your expected savings. It plugs into your Internet router, just like an additional computer.
• Your VOIP phones no longer plug into the wall. They must plug into one of two jacks on the gateway, one jack for each of two numbers, e.g. phone and fax. This means if you want extension phones in other rooms, you can’t use the in-wall wiring. You must string your own wires to hook the phones into the jack via a splitter.
• The VOIP phones don’t work at all if your Internet service is down or if your gateway has no power. This means they won’t work in a power outage the way land line phones do. In such an emergency, 911 or your burglar alarm phone would stop working.
• VOIP phones are incompatible with directly wired apartment intercoms.
• If you drop your land line voice phone service, the phone company will charge you more for ADSL service. If you also drop ADSL service from the phone company, and replace it with third party land line ADSL service, they will still charge you for a dry line (a raw copper line without any phone company services using it). This means your expected savings will be eaten up. Only if you get your Internet service from cable, you could drop your land line, and not have to pay the phone company anything. So your savings are badly eroded.
• When the phone is operating, it is using your LAN, chewing up some of its capacity thus slowing it down. However, a voice connection is less than 64K a second. What is more of a concern is your computer activities bogging down your VOIP connection leading to breaking up.
• If you need two lines, you will need two gateways and two sets of phones.
• If you use pure software VOIP, e.g. Skype, then the computer has to make the ring. It may not be loud enough to notice.
• If the computer is powered off, it can't take incoming calls, or ring, or make your phone ring. I am not sure what happens if the computer is merely hibernating.