For most of us, in daily life we are rarely exposed to things that poke, scrape, or cut us. On the rare occasion of a small injury, soap and water will be close to hand and probably a band-aid. For bigger problems, other people are there and help is only an ambulance ride away. In the backcountry, an untended scrape can lead to infection before you realize it, putting your health and possibly the safety of the group at risk.
Like many gear choices, you should first define exactly when, how, and where you will use your medical kit. If you trust your guide to have a complete setup, maybe all you need are bandages, acetaminophen, and handy wipes. But if you know that you'll need Motrin, an epi-pen or extra contact solution, this is where to put it.
So, you have basic first aid kit, or maybe something more substantial. But do you know what's in there? What is the difference between a butterfly closure and a Band-aid? Betadine and Benadryl? Luckily, many of the elements of pre-built medical kits are very clearly labeled for their contents and use. But if you don't even know you have butterfly closures in your kit, it doesn't really matter. Read the packaging carefully and peruse the contents list. The type size will be small, but as you do more research, you'll become familiar with the differences.
Building Your Own
For a quick 'done in a day' trip, the minimum you need to treat minor scrapes is the ability to clean, disinfect and protect. Alcohol swabs, antibiotic ointment, and self-adhesive bandages in a ziploc bag will get the job done, but those bags are not very durable, and the slick plastic is hard to keep hold of. For bigger emergencies or longer trips, a prebuilt kit in a red nylon zippered bag will be easier to spot in your pack, is designed to hold the contents in a way so you don't need to dig around, and can be reclosed easily.
Unless you have access to a hospital pharmacy and supply closet, it will be hard to find the small quantities and dosage-sized packaging necessary for field use, so a good prebuilt kit is actually cheaper than what you would spend on the large quantities you would need to buy to stock your kit.
Understanding the contents
When you get your kit home, find a clear space and open it up. If the first time you see the inside of your kit is when you need it, you'll be at a disadvantage. You don't need to unpack it, but at least poke around enough to know what you've got. Now is the time to add your favorite knuckle bandages, allergy remedy, or other personal items. If you will be the only person carrying and using the kit, make sure to include your emergency contact information and any specific medical needs or requirements you may have. In a pinch, this may be the only way someone else can learn this information.
Pick an 'anniversary' date for your kit to check your supplies, because opening up a vintage medical kit is rarely pleasant when you need the thing that has dry-rotted.
Whatever you decide, keep in mind that the value of quality will remain long after the satisfaction of savings fades.