I couldn't get my head around Haze or aMSN. The latter seems to be more of a web-link rather than fully integrated into the phone (but am not expert). I tried them and Pidgin and butterfly. Each is different, the latter two work well for me. Butterfly with the in-built N900 Conversations is easiest to use and update status. But Pidgin is pretty cool for seeing who's online in one window and has many plug-ins to expand functionality.

As I don't have dozens of IM contacts (billy no mates I guess), I tend to use Butterfly for ease of use.