I'm not a Lego gamer myself. For me the reasons are: too pricey, and too "toyish" (I like some toys in my gaming, but they have to fit reasonably well into a more traditional minis style). That said, I can certainly see why people do it. Legos have their own charm, they are tough and do not break easily, lots of stuff can be used cross-genre, and the company has come out with many nice sets recently. Add to that they would be a great way to introduce children to the hobby, and you have many potential benefits. Though I do not collect them myself, I certainly would not turn down an invitation to a Lego game.
Lego MonsterFighters, the Movie:
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I saw these in Barnes & Noble the other day, and thought I'd look them up on the web. When I saw the Lego website video, I thought it was great fun and wanted to share.
Note that this is definitely Transylvania-Hollywood VSF, with a character who looks like Elvis on a Victorian motorcycle, but the rest of the characters are more or less traditional (the retired british army colonel, the occult scholar, etc.) and the monsters are quite nice.
Within these limits, it seems to be a very good product for those into Lego gaming. You probably would have a hard time doing H. Rider Haggard or John Carter style adventures, let alone redcoats vs. Zulus, and I don't know that I'd quite go with the storyline they suggest, but for a Van Helsing vs. Dracula rpg, you probably could do far worse.
Anyway, enjoy.