Can the iPad 2 be used for everyday telephony? Is it possible to use the iPad 2 as a kind of oversized mobile phone for everyday use?


Using the 3G functionality available in some models?
Using VoIP through WLAN?
Using other means? 
With or without additional gadgets like headsets


and I mean not just theoretically possible, but really feasible with decent audio quality and a pleasant experience even for long calls. 

Does anybody have experience with this?
There are numerous possibilities outside Skype.

First; the iPad fully supports the three ring headset that Apple sells. Both the stock Apple Earphones that ship with every iPhone, and even the Apple In-Ear Headphones. Meaning that you can use third ring microphone for any/all input methods. And, of course, all iPads have built-in speakers and microphones, so you can even use it in a "Speaker Phone" style of use.

If you have access to a Asterisk or similar SIP PBX server, then you have access to a myriad of Softphone applications, many of which are very capable and support all the latest features (namely backgrounding) including 3G calling where applicable. Take a gander at AppShopper.com's list of results when searching for "softphone".

While not an official recommendation because I haven't used it personally, I do give a nod to Counterpath's Bria app, because I've used Counterpath's SIP clients on the desktop for years.