Backpacking rain gear keeps you dry and allows you room to move with your pack on. But you need more than a raincoat. You also need rain pants. When you are hiking in the woods while it is raining your legs will come in contact with brush. The brush will be wet and the water will be transferred to your legs by contact. Unless your pants are waterproof your legs will get wet and the water can run down your legs into your boots.
You also need a hood. The rain will run down the top of your pack ( even if you are wearing a hat ) and flow down your neck. You can bypass this if your backpack rain cover has a hood to bridge the space between your pack and neck but you will not have your pack on 100 % of the time.
There are many types of material that claim that their items will keep you dry. They may work for a while but after being out in a downpour all day with a pack on your back most fail the test. The reason is the same as touching the side of a tent. While the material will repel water, under pressure or in contact the material wicks moisture. This is amplified where the harness of your backpack contacts the rain gear.
The legs of your rain gear should also fit over your boots. If the bottoms of the legs are too narrow to fit over your boots they will ride above your boots. Eventually your boots will fill up. Your pant legs of your rain gear should also be big enough to be removed without taking off your boots. Zippers are useful for this because legs that are too big flap in the breeze and can be a hindrance in walking Velcro works but can become opened by branches and your legs need room to vent.
The only thing I have found to be waterproof is plastic and rubber. The problem is plastic and rubber don't breath. Your perspiration will build up under the rain gear and you end up wet. Once you get wet you get cold. This could be dangerous in cooler climates. In warmer climates being wet will help to cool your body.
You do not need a yellow heavy duty rain suit used by construction workers but your rain gear should have a layer of plastic or rubber somewhere. Rubber is more flexible than plastic and will last longer. The problem with rubber and plastic is a soft flexible layer is vulnerable to abrasion and tears by brush. So the layer of waterproofing should be protected by something.
The layer of rubber in quality rain gear will be between layers of polyester or nylon for protection. You have to accept the word of the manufacturer that the waterproofing is there because you won't see or feel it but will notice it in a downpour.
Because rubber and plastic do not breathe your rain gear needs vents to let out some perspiration. A large vent on your back is useless because your backpack will block it. The armpits should have vents and the bottom of the rain coat should fit loosely over the rain pants. Again, the backpack harness will help to seal the bottom of the rain coat. If your pack fly has a hood you can omit using the hood on your rain gear but it should be closed up to prevent filling with water.
If you plan on using rain gear in cooler temperatures it is wise to purchase sizes that can accomondate insulating garments underneath. If you buy insulated rain gear they will be too warm in warmer weather. It is best to buy backpacking rain gear as a shell and dress in layers inside the shell. to find out more about how to dress warm for cooler temperatures see my cold weather clothing tips.