While I applaud the development of bitcoin-related software, I think it's a horrible idea, just like the JavaScript miners. You're:
1. Potentially consumes all available CPU resources of your visitors unbeknownst to them (especially bad considering laptops and smartphones, which have limited power and small fans at full throttle are uncool as well)
2. Even if you have people give you full consent, they'd be better off running a miner closer to the bare metal. While probably a few orders of magnitude more efficient than JS implementations, it's still a waste of energy.
3. Having to load the Java VM is pretty annoying by itself.
Nice tech demo, but don't ever serve this to my browser or I'll never return.