They're very different. Mainly, modern Western RPGs are all about freedom, about letting the player shape the story. Japanese RPGs, on the other hand, are more about using the player to tell an amazing story that is more set in its course. Gameplay styles differ too, the most noticeable being that WRPGs use real-time combat: it's generally less strategic, but a bit more hands-on.
That said, you'll probably enjoy them if you get the right ones. But if you're looking for the epic stories of JRPGs, you'll be hard-pressed to find them in Western ones. I'd recommend Mass Effect and the Knights of the Old Republic games if you want the smoothest transition.
For the record, though, a Western composer has yet to even parallel Nobou Uematsu, so though there are some amazing musical scores, don't expect a soundtrack of Chrono Trigger or Final Fantasy VII level.
