I depends upon where you travel and what first aid knowledge you have. It's one thing if you are never too far from emergency care, and another if you go into remote areas (especially if you do hiking where you may be a day or more away from emergency help. And all the equipment in the world is useless if you don't know how to use it.
I generally think the store bought kits are expensive, and especially the cheaper ones don't really have much in them other than bandages, over-the-counter pain meds, and antiseptic wipes. You can often make up your own kit with items you already have in your medicine cabinet at home.
I would recommend the book "A Comprehensive Guide to Wilderness and Travel Medicine," by Eric A. Weiss, M.D. It includes an appendix on emergency kits.