My experiences with VikingTalk after one month. I'll start with a score of 10 and increase the score by 1 point for each advantage and decrease it by 1 for each disadvantage.
Installation: pretty painless, though confusing for non-techies. You have to install the app, then contact VikingTalk. They send an sms stating you have to remove any previously installed version and reinstall it. After that, you receive a second sms to activate your account.
After upgrading my iBrain to iOS 5, I had to repeat the entire process. So -1 for the rather complicated setup process.
Using VT over Wifi: All good as long as I have 3 bars of wifi. Any less will cause terrible calls with loads of hickups. No information seems to be available on how to optimize your wifi settings (QoS?) for VikingTalk. 0 points added/detracted.
Using VT over 3G: Good call quality from my side. Other side sometimes complains about call quality. +0.5
Using VT over Edge: absolutely impossible to use. -1.
Total score: 8.5
It's a promising application, but more info should be released on how to troubleshoot bad call quality.
Disclaimer: I suspect our Telenet wifi router to be absolute rubbish, but still, without any info on the protocols used/possible optimizations, it's very hard to draw a final conclusion.