They were quite popular because dooglus (who owns Just-Dice) recommended them after gambling restrictions in Canada forced him to stop accepting BTC and temporarily disable his site (it's currently back up and running but it now uses an altcoin called CLAMs). Before dice.ninja, dooglus recommended another dice site called Dicebitco.in which also closed down in similar shady circumstances. I lost a small amount of BTC from that one:
Man, that makes me sound bad. I can't really even argue with it, since I did speak positively about both sites. I played and invested on them both, and both seemed legit. I was always careful to say that I didn't know who owned them, and that I couldn't vouch for them in any way. But I did advertise both of them in my forum signature for short periods.