My four nephews watch Power Rangers whenever they can.
They also like watching Go-onger, which is kinda annoying when you're trying to do something and they're all 'oh there's the green car! oh who's that evil one?'. We watch it raw and sub too. One of them can recognize the OP and the ED, to the point of rushing to the computer whenever he hears it (even if it's winamp/audacious playing the go-onger single xD).
I had shown them before the first episode of Geki, but they didn't like it that much. I suppose the first episode of Geki isn't really something you'd show to the average 10-years-old with a short attention span. (lol run-on sentence)
Once Go-onger finishes and they're capable to read english subtitles, I guess I'll show them Boukenger. They can get the gist of it by watching it raw or english subbed, but it'd be awesome if they could understand the story. It'd certainly save me from having to explain whatever's happening on-screen, lol