Well I belong to all of them and get different opinion or support/help from each one but, from experience, xsibase and si-community has been the more productive, helpfull and active of them all on the Softimage front.It's not a respected industry "neutral" ground like cgtalk, or the official XSI mailing list. Personally I believe it would be best for the community and the engagement if xsibase.com finally died and people regrouped on cgtalk and/or The Area.
The Area site need some love, the frequency of post is less than SI-Community or XSIBase and the response is even less. many SI user refuse to frequent the Area for some reasons or other. As for the blog post, well you make your own jugement (a post every six month does not make it a blog IMHO, Thank SB for XSI Support blog). So the Area is not a good choice to gather at this point in time for me. Also, to login to get a free ice compound is a pain (yeah RRAY).
XSIBase is the most mature site but it is also the least functionnal of them all. To post a picture, compound or plugin require jumping some hoops. It is also plague by hacher attacks and many of the links to scripts, image or plugins are dead. I agree with Luc-Eric on this one, XSIBase need a serious revamp or its going to die. Too bad, it has a lot of activities and good information on the site. The people are very helpfull and a good representation of new and more experience users.
CG Talk is a mature and well maintain website. It has a lot of activities in other areas of the 2D and 3D industry. CG Talk/CG Society have competitions, weekly workshop and more. Unfortunately, on the Softimage specific forum, it's almost dead air (20 threads). Granted, if we were to move all to CGTalk, the activities would increased significantly.
XSI List, the grandady, I like it but refrain to post, it's more an attitude of listening and learning from the master of the industry. Can be intimidating for newcomers! Not recommended for an all out gathering, nuff said.
SI-Community started slowly but is picking momemtum. It may be a small site but it is modern, well maintain and active. It is easy to post compounds and plugins. It may be the yougster in the group, and it shows in some of the post, but given enought time, it will mature into a CGTalk or XSIBase. There is a good representation of users, althout I would like a little bit more participation from the pro!
What I am looking for is an active Softimage forum that provide a good amount of functionality to share information. The site must be easy to navigate and to search.
In the end, my preference are in this order SI Community, XSIBase (modernized if possible) or CGTalk (if the community become more active there).
My appologies for my long rant!