I would say not very promising but not a complete waste either. check here http://google.com/trends?q=showbox&ctab=1&geo=all&date=all http://www.sitefirms.com/domain-sales/box So for an interested show business client you may get lucky. However wait for few years to build up interest in mobi extension beyond non generic/short domain(it is a gamble anyways). regfee - low $xx if you want to sell it now(mostly to a reseller)
I agree ikunal. Its current value is regfee, but in time it can get you a low $xxx profit. But you need to be patient for that, until the .mobi will boom in the public conscience.