On 3G, it's perfect for Skype messaging and long distance texts; on Wi-Fi, it near-magically turns your iPhone into a VoIP handset.
Given that you can't make VoIP calls unless you're connected to Wi-Fi, Skype is surprisingly useful: even if you're tethered to your router's range, having a phone-shaped tool to make Skype calls is really nice, and its messaging service is a solid, not to mention fairly ubiquitous, way to keep in touch with people.
From The App Developers:
Skype gives you free calls and instant messages to anyone else on Skype. You can also call landlines and mobiles and send SMS anywhere in the world, at great rates. Skype is free to download and easy to use.