It mostly fits for people needing exotic mesh networking and not much else. "Much else" would imply just running it on a Pi.
If you want to play with the hardware you have to bite the bullet and get used to device costs being 2 to 10 times the cost of microcontroller or Pi unit costs. Contiki hardware is extremely expensive.
If you want to play with the software run emulators / simulators with contiki on desktop hardware, this cost is basically free (other than 100 watts per desktop etc).
If you want to play with mesh networks the best bet is something like WRT54GL with ham radio mesh software on it aka HSMM for $50 per node (at least its $50 Amazon Prime delivered, maybe more or less for you)