I like to hike in the national parks and use Trails Illustrated maps when they are available. Trail books often have small maps of the trails they describe, and those maps are fine for short hikes, but it's a good idea to have a larger topographic map for longer hikes. TI maps are durable and have a water resistant coating. (Good for when you get caught in the rain; probably less so if you fall in a creek.) TI maps mark the trails and their topographic features. The elevation markings give you a good idea of how steep a portion of a trail is. I've used TI's Yellowstone map for several trails and always found them to be accurate and helpful.
I recently purchased the 2008 revision. Unfolded, the map paper is the same size as the 1998 revision, but feels substantially heavier. The park map, however, is now a bit bigger and is split with the north half of the park on one side and the south on the other (with a generous overlap between the two). The written information about the park that was previously on the back is still present, but is reduced in size and located in several smaller boxes spread around the map. A new feature is that the information on the map does not end abruptly at the park boundary. Thus, for example, if you use the north entrance to the park you can see some additional trails, picnic, camping, and fishing areas marked to the north of the park as you come in on US 89. I don't use it, but the map now indicates that it is GPS compatible with a full UTM grid.
TI's Yellowstone map is worth having if you plan to do any hikes longer than a few miles. The 2008 is a good buy, particularly if you have an older revision that is showing some wear. If your older revision is in good shape (they are durable after all!) you can probably get by with it.