If you've used Skype on a high-end Nokia, you'll definitely look at Skype on the iPhone as a glass half empty. Skype's biggest shortcoming on the iPhone is that it only runs over wifi, not mobile broadband. The next biggest shortcoming is that, like all third party iPhone apps, it won't run in the background. As soon as you leave the Skype app you're logged out, so it can't lurk in the background waiting for incoming calls or chats.
These restrictions make Skype on the iPhone all but useless for incoming calls. If you leave Skype running and lock the phone, you can still receive Skype calls if you're on your wifi network - although after a while the iPhone switches from wifi to mobile broadband to save power. Now people can still see you online, but if they try to call you they'll get a "Call Refused" message. Unfortunately this is the exact same message they get if you decline an incoming call, so there's no way for your friends to know whether you're deliberately snubbing them or not. It doesn't even make a difference if you've got call forwarding turned on. I don't have Skype voicemail activated, but I suspect it wouldn't kick in either in such circumstances.
Running on a Nokia, Skype is tightly integrated into the phone system to the point where incoming mobile callers get an engaged signal if you're already on a Skype voice call. Not so with the iPhone. If you're talking to someone over Skype and a mobile call comes in, the person on the other end instantly gets "Call Held" - with no explanation as to why. If you answer the mobile call, the Skype caller gets cut off, but if you decline the incoming mobile call you're reconnected to the Skype call - assuming the person on the other end hasn't hung up in disgust.
I even tried putting the mobile caller on hold, relaunching Skype and ringing back the person to explain what happened - but the Skype call wouldn't connect properly while I had a voice call on hold.
If you can live with all of the above, the Skype app itself is fairly good. You can make Skype to Skype as well as SkypeOut calls and receive SkypeIn calls. I got excellent sound quality on both Skype to Skype as well as SkypeOut. From within the app you can also change your Skype status, edit your profile and view your Skype credit.
Clicking on a Skype contact gives you the choice of calling someone or chatting with them. You can also enter numbers with the dial pad or call up numbers from the iPhone's address book - although if you intend to use this feature you'll need to go through your address book and add the country code to each number. It would be great if you could set your default country and have Skype automatically add a country code.
The ability to make and receive Skype calls from the iPhone is handy, and the sound quality is very good, but the various limitations means Skyping on an iPhone is not all that practical.