Just wondering how far folks go to pursue copy-cats, and with what success? We'll all be aware of the oft-stated 'fact' that a game concept can't be copyrighted, but there are other means of protecting IP, quite apart from copyright. My understanding is that whilst that may be true, there are other legal recourses such as 'trade dress' that might be useful weapons, for anybody prepared to hire a suitably expensive lawyer! Alternatively I'd have thought that, whilst software patents are often seen a negative context, they might well be appropriate to protect a novel game-mechanic? I emphasise I'm not a lawyer and even what layman's knowledge I have would not extend to US laws where, arguably, such battles (over Apps) would likely be fought.
Just go see a lawyer if you are concerned. What can be protected is your game art/your specific code/your branding/your story Then there is of courses the costs of protecting it, although it is pretty easy to issue takedowns.
