Been using Fring since I got my iP4. A good alternative for those who need to video call someone not on iP4 and it works over 3G. To those wanted to use Fring, here are some points based on my experience with it:
1. Fring will populate your Buddy List with your iP4 contacts.
2. It is best if the person you wanted to voice/vid call also have Fring installed on their devices. If they do not, then Fring will do a normal cellular call (voice only) to them.
3. Fring will search your phone contact for existing fring users. This can be turn off if you want in manage add-ons (address book). Alternatively you can search by User ID (often time faster).
3. No, it cannot video call those without Fring unlike your Nokia etc.
3. If you're on wifi, then the call is "free" but on 3G you still have to pay for your data usage to your telcos, hence the 'warning'. (if you're on unlimited data plan, good on you).
4. Quality varies with connection speed on both sides.
5. The vid call will be "windowed" unlike Facetime.
Tango is your other alternative until Steve Job stops believing everyone on the planet is or will be on iPhone and enable Facetime to call non-iPhone users and work over 3G.
Cheers