Yes and No. Yes it's worth getting. No it's not a good place to start in the series. N:TW is very different from other TW games, cause it doesn't feature a full Grand Campaign and it's based on a semi-narrative around Napoleon's life. So it feels like you playing a documentary about Napoleon.
The other TW are based around a large period in history(The rise of the Roman Empire, the Dark ages, Medieval time and the gunpowder age) and these games feature a Grand Campaign with much more freedom of factions and choices. They feel much more expansive if you get me.
Napoleon is a great TW game, it feels very polished and it succeeds in doing what it's supposed to, which is awesome. But it's a TW game for long time fans.
Get Rome:TW and it's expansion first. The game is old but it's one of the best games of all time.(!!!11!!) It still holds up very well to today standards and there's tons of mods in TWcenter to download so it will last you FOREVER.
If you like Rome, do get Empire and Napoleon. They worth it.