The trick is as always to know what you are looking for and find the tool with the strengths in the right places. As with any system, Contiki has its strengths and weaknesses. The Coffee file system does its job well for what it was intended for (storing simple files on an on-board serial flash chip), but it seems like it didn't match what you were looking for. Filesystems certainly wasn't a design goal of Contiki, there may be better places to look.

If you're looking for a lightweight OS that does low-power IP networking with built-in support for self-healing wireless meshing, I think it is difficult to find an OS that beats Contiki. Take this from someone who has in fact built a business on top of Contiki :)