I installed Fring on my phone tonight. Fring is a free script which allows you to access all major messenger services through your phone. It allows you to chat and call via AOL, Google Talk, MSN Messenger, ICQ, Yahoo, SIP, AIM and Skype. It also has Twitter too!
The messenger service works pretty good. You can chat and call as you normally would. Some features are obviously missing though it does allow you to send files through it. The windows messenger live script I installed has a lot more features and is more suitable for using msn however Fring connects you to all chat scripts at the same time. Which is good and bad at the same time. I have some contacts on Google Talk through using gmail and I have less than 10 skype contacts. However, I have dozens of contacts on msn messenger and having all chat scripts on at the same time is obviously not smart.
You will get an alert for every message you receive and twitter updates too. The problem is, typing on a mobile phone is obviously slow and cumbersome so it isn't really suitable for chatting to more than one person at a time (even though it handles it pretty well).
The Twitter part of well is limited but it works really well. You don't see older messages, you only get ones which were received since you had Fring on. If you want to see older messages you simply login to the main Twitter site anyways so it isn't a big deal. So in this regard it works really well for updating your Twitter when your on the move. Which is pretty handy if your out and about and want to quickly let friends and colleagues what your up to and then log off.
I have set mine up so that Twitter and Skype are logged on when I switch Fring on. If I want to chat to a friend on msn it's probably more practical to use the windows messenger live script so that I'm not bombarded with too many messages at once.
I don't have an internet bundle packaged with my monthly contract but since I work from home anyways I'll only really use it when I'm stuck. My phone does have wifi though so I could always quickly logon when I'm at a friends house and chat for free!
If you want to add messenger to your phone, I'd recommend checking Fring out.