I've always been a big fan on mods for all games, but recently I have been adding many mods to my minecraft.
And I would like to tell about my experiences with the different mods as well as hear others experiences with mods for minecraft.
I'll start out with the 2 mods I use the most.
Single player commands, a singleplayer only mod (duh, its in the name xD) which allows you to use various commands in game to for example killing every mob in a radius of 1000 blocks, or "fixing" water so it fills an certain area with water. Personally I only use the mod to change the walking speed as well as how high I can jump, makes building tall buildings so much faster, I also use it to disable damage from mobs, fire and water. And so far I haven't met any bugs with this mod so I would recommend it to others who might be interested.
The second mod is called "Too Many Items" and is an in-game inventory editor which allows you to spawn every item in the game, individually, in stacks of 64 or in infinite stacks at 111 which never runs out no matter how much you use them. If you use this mod together with Single player commands you can cut down the building time on a building project with at least 33% (don't thrust the %.. I suck at math
.. but it will be cut down) and makes sure you wont get killed by a random creeper in the process.. There is few bugs I have encountered with this mod, one is that if you accidentally drop an infinite stack of blocks down into a 1 block hole and places a block there you wont be able to pick up the infinite stack again for some time. and another bug is if you don't have a modloader (which name I have forgotten) in the game you will experience your infinite stacks being.. finite until you open your inventory, making the stacks restock as long as they are above 80.