I've played many musket mods for Mount & Blade, and always thought they were neat but not worth getting into. Napoleonic Wars has outdone most of those mods and done some unique stuff. Having played about 10 hours so far I thought i'd share some tips from my experience, most should be fairly obvious but hopefully they'll help someone.

• Always be in cover- This is pretty obvious but should be noted no matter how inaccurate bullets are in the 1800s, sitting in the open is simply a waste of your time. When you are in the open it should be because you're moving to another piece of cover. Bushes, trees, even a small rock that you can use to cover up part of your body can be usedful. Stay hidden whenever you can. This is realistic 19th century warfare with a 21st century mindset, you should behave like a guerrilla, hiding and popping out to take a shot. Not standing in the open like you're at Waterloo (Unless you're in a line Battle event, obviously)
• The bayonet is strong, but the sword is stronger- What struck me when I first started playing was that most of my opponents had seemed to forget their Warband roots. Even though swords aren't quite as agile as they once were they still have numerous advantages over the bayonet, they're faster, can attack at multiple angles and can be used in small spaces. Sword Bayonets can be a bit too short to be effective by themselves, but any sort of sabre or straight sword is worth using. The bayonet can be a one hit kill but if you miss you're stuck in the animation for sometime and it can only attack forward (Up or down) which is easy to block. It makes handling a group of enemies a real hassle. Remember what you learned in warband and take advantage of your sword whenever possible.
• Troop type matters- Each troop has it's own stats and superlatives. (Which are listed here) Rifle only classes such as the Jaeger have really high accuracy but are limited and number and lack bayonet weapons and good swords. Engineers can build things but lack any good offense. Dragoons are more versatile as they are mounted infantry, but are weaker all around. Heavy cavalry takes and dishes more damage but is slower.
• Don't fire into melee- Most servers have team damage on, so be careful with your shots, if you think you might hit a friendly, don't fire.
• Water is a threat- Between Warband and Napoleon, bodies of water have evolved to the point where you can actually drown in them (No more pretending to be a submersible by walking underwater for 10 minutes), crossing a deep river is always a bad idea, look for a ford or a bridge. Not only can you drown but water will also ruin all your cartridges, with the exception of the one you have loaded in your weapon. Earlier today i got caught with my pants down when i rode across a stream as a dragoon, encountered an enemy and realized I had no ammo left after firing my first shot.
• Pick your battles- Try not to get in a fight with more than 1 enemy at a time, melee weapons are slow enough that i's always preferable to only face one enemy at a time as I said above, bayonets are bad against groups and swords aren't perfect.
• Use dropped weapons to your advantage- Loaded muskets are still loaded once dropped in a close firefight, picking one up rather than reloading can get you an extra kill and surprise your opponent. If you decide to be a musician or flag bearer, you still have room to carry a rifle and cartridges as well as your flag/instrument. Also, trading a cavalry musket for an infantry musket with a bayonet can make you more effective later on in the battle.
• You can Fife if you want to- Musicians, officers and Flag bearers can create buffs to boost their team, this concept really isn't explained by the game but its important. Drums aid reloading, fifes and trumpets aid marching (I'll check that), flags enhance melee and officers can enhance accuracy. When you have a flag you can stick it in the ground by switching weapons and create an area of effect where melee is enhanced for your team. With an instrument just right click and choose a tune, you can also select "enable play together" to get in rhythm with other musicians. Buffs are server specific, some servers will turn them off because they can cause lag (Hopefully they'll patch that)
• Some extra things- Hitting C will make your character yell a warcry, Ctrl-J makes your horse rear up dramatically,

I hope some of this helps!