i have several first aid kids that i have picked up from various places. One thing that i always do is open them and inspect the contents. Why? Firstly you dont want to be searching for things when someone is bleeding out. You should always be familiar with it before you need it. Secondly i have often found that they are missing quite essential items. Most of my kits only come with one bandage, one set of gloves and one non stick dressing. Now if im at the scene of an accident i know i will need several of each of these. Non stick dressings especially. I also find the tape they include is hopeless at sticking anything. I much prefer sports tape which can be used for securing bandages rather than messing around with safety pins.
They also dont come with face masks if you have to give cpr. One hazard with cpr is that the person can vomit while you are trying to do it. You dont want that in your mouth! A face shield is a must, you can get them for your key chain now too. I also like to throw in some pain killers incase help is a while away.
And remeber, pack it before an emergency!