If anyone is under any misconception that the musketeer is a good unique unit, they should consider that the Samurai is only 1/6th more expensive, comes in slightly earlier, and weighs in at 4/4/2 to a musketeer's 3/4/1.
Samurai are only considered a slightly-better-than-average unique unit by most, and yet are clearly far better than musketeers.
@Charis: yes, there does seem to be some correlation between weak unique units and strong civilization traits, but this is all very murky.
To start with, it's not clear that Firaxis does consider some civilization traits 'better' or 'worse'. What's more, some civilizations seem to get an awkward pair of traits, with a poor or useless unique unit, e.g. England, Russia, and the Mongols.
Likewise, some civilizations get a powerful set of traits, and also get a powerful unique unit; e.g. the Persians and the Chinese.
I'm starting to think that the traits/unique units distribution isn't in any way connected: Firaxis decides upon them seperately, without consideration of balancing poor unique units with good traits and vice versa. There's also the consideration of actually providing traits and a unique unit which does match up (vaguely and a little arbitarily) with the civilization historically.
Of course, Firaxis does test the game most thoroughly on Regent level, and I think that they consider a late unique unit just as valuable as an early one.
-Sirp.