We have both.
One of the advantages with having a land line is price. When we just had two cell phones (we now have my FIL on our plan), we were spending about $70-$80 per month and we would wind up going over our minutes. We have Vonage at home and it's $31 per month with unlimited minutes, call waiting, three way calling etc.
The other thing I like is that we don't have to be bothered at work on our cell phones with phone calls that are confirming appointments, friends calling, businesses, etc. With cell phones, you the owner, are paying for in-coming and out-going calls. With land lines, you are not paying for incoming calls. And you can have a phone in more than one room without having to pay an extra fee each month.
The good thing about having our cell phones is that if the power goes out, we can still use it and it definately comes in handy in an emergency or if we are running late to someplace.
But I would never just have a cell phone only in our house. Mine would always be on the charger downstairs and I wouldn't hear it ringing if I was upstairs.