Pros:

• Open source. For some this doesn't matter as much as it does for other. For me this was the primary reason for moving to Identi.ca from Twitter.
• Most of Twitters features already implemented. Those who use Twitter should feel right at home with the same syntax and everything.
• Groups, this is a feature Twitter doesn't have and really should. It allows you to post to a group using the !groupname syntax. Everybody subscribed to the group gets sent your 'dent'(like a tweet). This means that you don't even have to have many friends on Identi.ca to make the most of it, just subscribe to groups of interest.
• OpenID. You don't even have to get an account to try it out, whats the excuse?
• Twitter integration. If you have backwards friends on Twitter you can still keep then up to date.

Cons:

• Less people. It's currently less popular than Twitter. However Twitter wasn't very popular when it started.
• Currently it is only possible to push to Twitter and Facebook. Laconica cannot yet pull messages from either of the two sites.
• Fewer clients. At the moment there aren't as many client applications as there are for Twitter. However this is becoming less and less the case because of it's Twitter API compatibility.

Conclusion

Sign up or use your OpenID to try it out. We should start a Geeks & God group :)