Skype has a mobile version of it's IM application. It is pretty huge - 5 MB download size and 3-4MB memory when application is in use. Also, it supports only skype protocol (no google talk, yahoo or anything else). So, it loses the edge when compared to its competitors here.
Basic text chat, voice chat, skype-out calls all work fine. But the application itself seemed to be a little slow (when compared to others).
This is a small IM application that supports many protocols - Google Talk, Skype, Yahoo, MSN, Facebook, twitter, etc. You can even do voice calls on all these. I did most of my audio calls on skype. There was a wee-bit of delay. Not much, but we could make out that there was about half a second delay between when we talk and the other party hears. Sometimes, it was annoying. But it was mostly OK. And the audio quality was good. Making Skype-Out calls was also very simple. All-in-all a good application. One disadvantage is that it only supports Skype and no other SIPs (like Gizmo or others)
This is one of the best applications I've used. Similar to Fring, it supports multiple protocols - Skype, Google Talk, Yahoo, MSN, Facebook, twitter, etc. It supports text chat as well as audio/video calls on all of these. Unlike Fring, there was no noticeable delay in audio calls over Skype. The audio calls all sounded great. The contact list is also well structured with icons indicating where the contact is from (GTalk or Shype or Yahoo). Nimbuzz also supports other SIPs apart from Skype. You can configure it to connect to Gizmo and other SIPs easily.
One problem I noticed was that I could not make skype out calls easily. On skype/fring, I can call toll free US numbers using skype-out. On Nimbuzz, it told me I had to buy some talktime before I could make a call.. :-(
For now, I have all 3 installed. I use Nimbuzz for most purposes. But if I need to make skype-out calls, I use fring.
On 3G network, the call quality of calls over voice network and the audio calls over skype (data network) are pretty much the same. It costs me 30-40 paise per minute for a skype call. That is cheaper than my local call (40 paise per min). When I talk to my uncle in US, it is way cheaper. An ISD call costs Rs.7.20 per min.
Oh yes, I can do the same skype calls using a computer (maybe for same cost 30-40 paise per min or even lesser). But being able to use my mobile phone anywhere to call via Skype/GTalk is real convenience.