I have this weird feeling of cognitive dissonance regarding mods.
On the one hand, I love how much work modders put into a game and how that fills the game with content that is compelling beyond the original "vanilla" game. Some of it changes the way you play, which can add untold hours to your game time.
On the other hand, the amount of influence I exert over a game, the more I feel less compelled to finish it.
Let me describe an example. So let's say I add a spell to one of the stores in Skyrim. Now, I buy the spell and discover that it kills virtually any enemy in the game with little effort on my part. Now, I can choose not to use the spell. I can remove it from the game. I can keep it and just enjoy the game knowing that I've sucked all the challenge out. However, all of these choices lead me to the inevitable conclusion that there is something that dulls every enemy encounter in the game. Every time I run from a powerful enemy, I'll ask myself if I should be using that spell, and that conflict concerning my influence rears its ugly head again.