Aside from the "Hey, look at all those colorful little people I can see" factor, online collaboration via Sugar on the XO is a bit underwhelming in practice. There are a couple things to keep in mind, though.
First off, there is no question that collaboration in particular is way buggy and inconsistently implemented. The chat client has a harsh scrolling bug that makes is essentially unusable in the stock build (that's what happens when you decide to use an immature canvas object as a cornerstone of your UI), Write tends to crash when you switch to another activity, etc. That stuff will be fixed.
On the other hand, connecting with "the world" via a central Jabber server like xochat.org just isn't the use case Sugar was designed for. Each school will have its own Jabber server. Eventually, students will probably be able to view students from multiple servers. This capacity is built into Jabber ("s2s") but not implemented in the Sugar UI.
Regardless, however, if you're talking about collaboration within a school, and probably a small school by US standards (under 500), you can make a big assumption that one doesn't make about collaboration over the internet in general: that you can assume a lot of "out of band" communication, that is, the kids have ample opportunity to meet and discuss things face to face in addition to via the computer. So when a kid is, say, sharing a document, it is pretty likely that he or she has discussed it with the students he or she will be working with, and they're expecting it. This is important because when simply sharing a Write document, it is only available via Jabber (or the mesh) while it is being edited. Compared to a wiki, this is a bad strategy for internet scale collaboration. Inside a classroom or a village, it makes more sense.
Notification seems to be a big weakness of Sugar right now as well. For example, if you decide to join a random shared Activity on xochat.org, it usually seems like nobody is home. That's because the person sharing the activity has probably gone on to doing something else, and doesn't notice you've arrived. Or you invite someone to a chat and they don't know it until after you've given up. This is because the only notifications come in the lower left hand corner of the frame, which is not visible while you're working in an activity. This is by design. You want kids to collaborate, but you also don't want them able to constantly distract each other. I think unobtrusive notification is a good choice for the main use case. It just doesn't work so well if you're trying to find someone to play Connect Four with.