1. "it's laggy running ubuntu - what's the big deal". It's running Ubuntu with the dual-core ARM CPU on board, not the Epiphany chips. The guy is demonstrating that the boards they are shipping allow for a user friendly environment for which you can jump onto and use Eclipse to write code for the multicore chips. This has NOTHING to do with the multi-core chips themselves, and has no intent of demonstrating the power of the "supercomputer" part of the board.
2. "Why would you use this if you could just use a GPU - they're really parallel right??" - GPUs are very very different beasts to CPUs. They are great at what they do, but they are tailored for very specific problems. Look up SIMD. A tonne of general purpose programs which need, for example, a simple 'if statement' quickly break down under SIMD.
3. "This will be great for mining bitcoins" - yeah. but you can do it on a GPU so stick to that. As far I can see (and why I backed the project), this board will be great for those problems which are not immediately or easily implementable as a wavefrontable algorithm for the GPU. I'm hoping you can just write a c program utilising pthreads which will be run on the Epiphanies cores