There are two different mods contained in the zip file.
The modded ee.class will make gold tools work as fast as diamond tools, and last about half as long. Gold tools will not be able to mine obsidian.
The modded ce.class file makes all tools last about 1.25x as long as they used to. Wood Tools: 33 > 41 Stone Tools: 65 > 81 Iron Tools: 129 > 161 Gold Tools: 513 > 641 Diamond Tools: 1025 > 1281
You do not have to use both at once, they work independently of each other.
Remember to remove the meta-inf file before use.
Other mods involving these files will break, and more likely than not will cause Minecraft to get a black screen until you either revert to vanilla, put the other modded file back, or get rid of whichever mod is conflicting.