They had an early heavier period, then went commercial for a bit then back to metal somewhat. They tended to have a quirky science fiction vibe in their lyrics.
Well judging by their lack of truly breaking through and having the staying power like Ozzy, Priest or Maiden, kind of like Thin Lizzy in a way, BOC didn't appeal to enough people.
They may not be your thing but if you haven't heard too much of their stuff then a good place for a metal fan to start is the Extraterrestrial Live CD.
It lacks original drummer Albert Bouchard for the most part but is a good rocking cross section of many of their tunes.
I so love google image search. The stuff that pops up when you search for an album's name.
That is one of mine too, Pip. The first album is great from start to finish. It just has this killer vibe, like some weird movie or something. I almost mentioned it in the post above as the second thing to check out after Extraterrestrial Live.