Checkout HipChat. Like IRC and has a desktop and web client. They also have API which you can use to post messages to channels from external sources. We have our Capistrano/Ant scripts post their status to our "Deployment" channel. Currently working on a bot that hooks into Jenkins so we can build via our "development" channel.
We use Hipchat at work at the moment and while I like it overall there are some issues that they need to address.
The Air app for desktop can sometimes have performance issues for no particular reason. They're about to release a native OS X app though so that should solve this for a large amount of users.
The android application in woeful. It consistently logs me out and forgets my password which means that it's totally unusable for out of office notifications from people. I've had to rely more on their email notifications for unread personal messages than their Android app. It's a real shame because I could see myself getting a lot more use out of Hipchat if it worked well enough.
The web client is a bit "special" though - after a day or two of running it with 4+ channels open the history will build up and jquery functions will start freezing the interaction. Flash/AIR client is much better, but... it's Flash/AIR (copy/paste issues, different set of fonts, message notifications not clear).
The best way to access hipchat that I've seen so far was just a jabber client - xmpp seems to be the backend of their service.
On the mac, at least, I've found the AIR client is really, really good. I haven't really run into any of the issues you mentioned and have been using it daily for several months.
And my coworkers really love the web client. And some use xmpp. Many of us regularly use the iOS app. I guess that's one of the strength's of hipchat, the variety of access methods that all feel basically first-class. (and one person on our team even uses the SMS integration.)
