I haven't had a landline for around 5 years -- as soon as broadband technology (cable internet, specifically) was available in the area where I lived in my college apartment, I ditched the landline and got a cellphone, as my only need for keeping a landline was gone. I make a lot of long-distance calls on my cell, but since I generally make my longer phone calls in the evening or on the weekend, when I'm not working, (using unlimited night and weekend minutes, of course), I'm happy with my current setup.
If I needed to make a lot of calls during the daytime, though, I would take the money that I would spend on VOIP hardware and service and put it into a cellular plan with tons of minutes. That way, I would have one phone number for all of my calls while being able to make long distance calls both during the day and in the evening, whether I was at home or not, with superior quality to VOIP.
You can generally get plans with thousands of anytime minutes without spending a TON of money (compared to paying 2 phone bills, buying all the hardware for VOIP, etc.), and with all the extras like free incoming calls, early "night minutes" start time (Sprint offers a 6pm night minutes start time on a free incoming calls plan for like $10 extra), free mobile-to-mobile, you can avoid even dipping into the anytime minutes very often, unless you make a lot of business calls during the day or something).
There *are* unlimited cell phone plans, but they are closer to $200/mo., where you can get a plan with a few thousand anytime minutes with a lot of the extras mentioned above for around $100. I'd rather spend more money for the convenience of one phone number and the convenience of making and receiving calls whether I was home or not.