I gave the first series of Toaru Majutsu no Index a whole 10, but I'll give NT a minus 1, to a total of 9.

Story : 10
It's still as epic as ever. Kamachi Kazuma never fails at giving his readers an epic and complicated story. The war between science and magic at the scale of the world is still masterfully written. NT is still one of the LN with the best story aspect.

Art : 9
The art is great, but there are some (not much) LN with better illustrations. So I'll hold back a little and give it 9 for art.

Character : 10
Accelerator is the best!!! Kamijou and his Imagine Breaker is just as cool, Hamazura is also funny as ever, and Mikoto is still my favorite. Where else you will find a Light novel with four main characters, each with their unique background and story?

Enjoyment : 7
What!? Why!? Here's the reason : it's hard to read. It is necessary to have a superb imagination and understanding in order to recreate the scene in the story inside your head. There are several reasons for this :
- Lots of terminologies and explanations. One point that will make you confused.
- There are too many new characters introduced in each volume. Another point towards confusion.
- The abundance of main characters actually became a drawback here. The story ends up unfocused, and if you haven't read the first TAMNI you might even think that Touma (the real MC) is just a side character. Yet another point towards confusion.
- The story switches between point of view too often. First two pages you will follow the MC, then three pages from the enemy POV, and the next page suddenly switches to some soldier in a battlefield. Total confusion.

Long story short, it's a great, VERY great Light Novel. The only catch is that you need lots of patience and imagination to read this. NT is actually one of the two most difficult LN I ever read (the other being Kyoukai Senjou no Horizon).
So this is my conclusion : Unless you have confidence in your bookworm skill, or a fan of Toaru series, I suggest you to think twice before reading New Testament.