I agree with all the previous posts. Only things I would add are the cost savings when considering battery replacement and the amount of space that accessories take up. With egos, mini, etc. you have to replace the whole battery unit when they die. With mods, you keep the body and just replace the (generally inexpensive) battery itself.
As to space savings, I only carry a small clip on pouch with two spare batts, 3ml bottle of liquid, and a spare atty/driptip combo. The pouch is smaller than my mp3 player. Back when I used egos daily, I was carrying a pack with two spare ego batts, attys, and a bottle. It was three times the size and was not particularly comfortable to wear/carry.
Mods are just battery housings in general. Some have extra nifty features like variable voltage and whatnot. All they are is a way to custom pick the features or form factors that suit you.