you can add some small mods and the game will get better, but it is easy to add a mod that changes the game massivey.
i recomend small mod or mods that round out the content i.e. H.E.L.L.U.V.A. Armor Completion Set, not a large landmass, 9000 quest 400 weapon and dragon mega mod.

the more mods you add the more thay clash with both each other and the game.
ever seen master chief fight predator and link in balmora? i have it kills the mood, dont let it happen to you.