The line quality is ~ 95% as good or better than land line. I find the quality far superior to Skype.

What you should consider is :
- The quality of internet connection (I use cablecome basic plan 3500). Usually you can use the highest quality rates without stress.
- The quality of your equipment (I use Sipura/Linksys/Cisco and they're pretty good)
- The quality of your providers. I find them satisfactory overall.

Reliability is probably the thing that may worry you, if you have frequent internet outages, etc.

My point is that I have mobile if I really need to make a call and have no internet.

The advantages of paying Sfr 0.00 for any fixed-line call worldwide, compensate all the "risk".

Cheers