I just did a quick test of the four major VoIP apps available for the iPhone. The test consisted of me connected via wifi (to my 8/.5 cable modem connection with 33ms ping to Seattle) using the app to call another phone and talking to myself. (It's not an exhaustive review--for full information regarding calling prices and international rates, you're going to need to compare the rate plans on your own!) I tested each setup a couple of times to ensure a single bad connection didn't skew the results. Here's what I came up with:

iCall
Pros:

• Allows several different kinds of accounts, including free ad-supported calls or calling plans that allow incoming calls (strangely, it's hard to upgrade once you've opened a free one--maybe the native Windows app makes it easier)
• Pay-as-you-go offers the best per-minute rates for domestic U.S. calls of all the apps (except for Vonage; see below)
• Your caller ID shows up on the receiving phone
• Good call quality (PSTN user hears iCall user crystal clear, though PSTN users sound slightly tinny to iCall user; quality supposedly better if you upgrade to one of the paid plans)
• Decent delay (~400ms)
• Full duplex
• Quick call set-up times

Cons:

• Free calls are (sometimes) prefaced by a short ad and are limited to 5 minutes
• Incoming calls not available on free account
• No free calls between iCall users (you can't view other iCall users or add them to a buddy list)
• App takes a while to load and connect
• Does not support push notification of incoming calls; however, it does switch to VoIP if you receive an incoming call via iCall's network (that gets forwarded to your regular voice number) and then open the app while connected to a wifi hotspot
• Not updated to take advantage of VoIP over cellular data network

Truphone
Pros:

• Incoming calls from PSTN (not clear how to set up, though--may need to be on a subscription plan)
• Free calls between Truphone users (and other interconnected VoIP providers via gateways, such as Google Talk, though I can't find a list of connected networks)
• Innovative "Truphone Anywhere" feature: if you're away from wifi, Truphone automatically dials a domestic gateway via the PSTN (no complicated calling card codes) and then completes the international call over their network so you can take advantage of [almost-as-]cheap international rates (this benefit will be obsoleted when apps get updated to allow VoIP over cellular data network)
• Your caller ID (sometimes) shows up on the receiving phone
• Excellent sound quality and very low delay between two Truphone users
• Decent call quality to PSTN (PSTN user hears Truphone user crystal clear; PSTN users sound slightly tinny to Truphone, though slightly better than iCall)
• Mediocre delay to PSTN (~5-600ms)
• Full duplex to PSTN

Cons:

• 5 cents per minute (more expensive than other services)
• Medium call set-up times (not as bad as Skype)
• Sluggish app performance; call audio occasionally stutters
• No push notification of incoming calls (despite the app being push-enabled!)
• Not updated to take advantage of VoIP over cellular data network

Vonage
Pros:

• Domestic calls appear to be free with no restrictions! ^
• Good call quality (both sides sound very clear)
• Decent delay (~400ms)
• Full duplex
• Quick call set-up

Cons:

• No incoming calls
• Outgoing calls appear to come from a blocked number
• Not updated to take advantage of VoIP over cellular data network

Skype
Pros:

• Incoming calls from PSTN (with subscription)
• Cheapest unlimited domestic calling option
• Most popular VoIP service with a wide network of users
• Calls between Skype users free
• Excellent sound quality (wideband codec?) and very low delay between two Skype users
• Distributed architecture is (allegedly) resilient and fail-resistant

Cons:

• Calls appear to come from random, unknown numbers (why on earth would a call be routed via Florida to call Alaska?!) unless you've set up caller ID (Skype charges you to set this up, and I haven't been able to get it to work anyway)
• 3.9 cent connection fee for PSTN calls (makes the decent rates less appealing)
• Charges more for calls to Alaska
• Long call set-up times
• Worst delay (~600+ms) to PSTN
• Worst sound quality to PSTN (sounds muffled and choppy to PSTN user, though it sounds decent on the Skype end)
• Not full-duplex to PSTN
• No push notification of incoming calls (app must be open)
• Not updated to take advantage of VoIP over cellular data network

So, here's how I'd rate them (for domestic calls; international rates may affect your decision):

Domestic outgoing calls only:

1. Vonage
2. iCall
3. Truphone
4. Skype

Full-featured app with incoming and outgoing calls:

1. iCall
2. Truphone
3. Skype

VoIP-only calls (Internet device to Internet device):

1. Skype
2. Truphone

If I'm running out of cell phone minutes and just need to make domestic calls, I think my first choice would be Vonage, given that it's both free and has the best call experience (good quality and low delay).

If I need a full-fledged setup that allows both outgoing and incoming calls, I'd pick iCall (either their Pay-As-You-Go or Total Access plans, depending on my needs), given their good domestic rates and decent call experience.

Truphone's sluggish performance, slightly lower call experience, and higher cost ranks it below iCall for the full-featured set. The added benefits of free calls between Truphone contacts and interconnected wider VoIP networks might be useful to some people.

For calls to and from the PSTN, I'd rate Skype last. They used to be the only major game in town for PC-to-PSTN calls, but their per-minute rates have gone up (especially since the advent of the connection fee; they still have the cheapest unlimited option), and their call quality is relatively bad.

For pure VoIP calls (i.e. not involving the PSTN), though, I'd rank Skype at the top. It might not be as open as Truphone (with the gateways to other standards-compliant networks), but it has a wide user base, and if your friends aren't on it, it's pretty easy for them to set up.

Hopefully these apps will be updated soon to allow VoIP over the cellular data networks and also to support push notification of incoming calls (so they don't need to be running to be useful). If one of them did this in combination with a dedicated incoming phone number that I could give out, I'd probably drop my cellular minutes package to the lowest level and use the VoIP service for most of my calls. (Don't tell AT&T! ) I think iCall (with its Total Access plan is the closest to being ready for that kind of a leap--they just need to add VoIP-over-data (VoIPoD? ) and push notification.