VOIP is a great thing when properly sorted. I have not heard of naked dsl being any different to regular dsl, as it is still working at the same frequency just without the underlying pstn service, but I could be wrong I've been out of the industry for a couple years.
There is a couple of catches for home users:
1) If you have a monitored alarm system you can not send modem signals through an adsl router
2) If your dsl link goes down( and lets face it computer networks go down alot more often than POTS phone networks), you lose your ADSL link and phone. You will have to call your ISP from a mobile or some other service to log a fault ticket.
So keeping your underlying pstn service is a smart thing to do, the ISP's node phones are getting better all the time, I have heard of quality of service (QoS) issues and consistent drop outs, but know of others that beside the differing tones cant pick a quality difference
Skype is good for home video/voice calls and the price is right $0
, for skype to skype calls. They also offer really cheap skype to Land line services worldwide
A 50% reduction in call costs sounds pretty good to me