The vibrating tattoo will allow you to never miss a call or email, avoiding the problem of not hearing or feeling your phone when it rings or vibrates. The use of ferrofluids for tattoos may lead to more practical applications of magnetic liquids.
The tattoo has limited marketability, without a lot of crossover potential. Employees will likely balk at the idea that their employers can keep track of them through a tattoo, while the mainstream market will likely regard it as a curiosity at best. A number of end user problems will need to be addressed even to bring the tattoos to market as a novelty.