In Fallout 3, I've been playing with the 20th century weapon mod. It's helping me appreciate the weapon decisions made by the game designers. Where 20th century weapons is more detailed, the main game is representative. So where 20th Century Weapons gives you the accurate maker and ammo type of a weapon, the base game gives you Hunting Rifle and fairly generic ammunition. The end result is that 20th Century Weapons is a myriad of weapon and ammo types, while the base game gives you a far shorter and intuitive list of weapons.
In my book, the designers made the correct call. The ability to sort out your equipment, especially for a novice, is more important than historical accuracy.
Another issue that I find is that 20th Century Weapons are historically accurate, which is a problem. They aren't in line with the game's generic weapons.
Interestingly, there is a pre-planned scene in the game where a Brotherhood of Steel squad helps you to fight some supermutants and a supermutant behemoth. The Brotherhood plowed through the mutants, pillaging their 20th century weapons along the way. By the time that the Behemoth showed up, the Brotherhood troops just about mowed him down on the first pass. Without a doubt, this mod greatly changed the balance of the game. In the end, that's a dealbreaker for me. That balance isn't BAD, it's just not what I want.
I'm on the fence about flavor. Fallout 3 is set firmly in a number of aesthetics, and far too many modern weapons break those aesthetics. In some respects, its a breath of life, and in others, it's a firm annoyance.