In the original 10 essentials, a map and compass were the very first items. They were taken together as single item, map and compass. The fact they were the first thing on the list is evidence of their importance. If you ever get lost the most important thing will be to find out where you are. A map of the area where you plan to be hiking is part of your preparation. A compass should be standard equipment.
Paper maps are available from a variety of sources. Most of them are based on the US Geological Survey maps that have been created by United States government for decades. National Geographic has a collection of comprehensive USGS maps for every major national park in the United States. Tom’s Maps are very popular among hikers and backpackers. These maps can be purchased from online sources like Amazon.com or eBay, or from a retail store like REI.
You can also produce your own maps for areas you plan to visit. You need supporting software for your computer and reliable printer. Special paper that is waterproof and can stand up to high winds is available on which to print your own maps. Some paper works with inkjet or laser printers, but other papers like Mylar work best in inkjets. This is because the paper can melt in the fuser of a laser printer.
A good GPS from a major manufacturer like Garmin, Delorme and Magellan will generally have access to very good maps. The maps are stored within the GPS, and in some cases are included with the GPS itself. Garmin sells its maps as an add-on, but Delorme includes its own highly detailed maps as part of the price of the GPS. Magellan comes with a very good base map, which is not very detailed at close levels but a subscription to National Geographic online maps is available.
You can use your smart phone as a reasonably good GPS. Several apps are available for the Apple iOS platform. Delorme Earthmate, intermap technologies and Trimble are very prestigious. They are available in the app store. Earthmate and Trimble are also available for android.
A map and compass are still considered preferable, because they are very reliable. Few moving parts, and no need for batteries. Handheld GPS devices and smartphones are very complicated internally. They run on batteries, which can run down, or they can be damaged to the point where they might become useless, for all practical purposes. A dead battery 15 miles out in the wilderness makes your GPS into dead weight. There are options for charging a battery, but we will talk about that later.
Consider taking a course in using a map and compass to find your location. If you ever get lost, this can save your life. A map and compass are not very bulky and therefore not a major burden in any backpack. Even though their use is cumbersome and time-consuming compared to a GPS, they still make an excellent backup. Then, get a good GPS and learn to use it as well.