Everlast punching bags are definitely a top-of-the-line product. They've been used by professionals for years and years with much success. I, personally, use an Everlast bag for my own training. The Everlast punching bag can improve coordination, strength, endurance, and can be used as an effective tool for burning calories–though, many boxers aren't concerned solely with burning calories.
The leather bags will last for years and years, and can be passed down through families and still look and feel excellent, due to their superior design. Leather Everlast bags tend to be more expensive, but they are well worth it for the style, functionality, and durability. I have never heard of anyone damaging a leather Everlast bag to the point that they need to get a new one.
And the canvas bags are just as great, though they tend to cost a bit less. Because they're made of rough canvas on the outside instead of leather, if you punch on them for a long time, you can wear down your gloves and mits through consistent, long-term use, though that would speak very highly of your training habits.
With the bags, some settling may occur if you do not use them for several weeks, but this is unlikely and if it does happen, it's easy to fix. One would need only pound at the bag for a little while to soften it back up.
Everlast bags come with high-quality chains, as well, so you can be assured they will not be easily knocked off the hook or out of the ceiling, or a combination thereof. Everlast punching bags can be opened up and replaced with your own filling, if you want. Some people choose to take out the fluff from their bags and fill it, instead, with sand or even small beans.
Something interesting that I learned about the Everlast punching bag is that they also serve an alternative purpose of being taken off the hook and carried around for a satisfying twist on your usual strength training routine. Trainer and fighter Ross Enamait uses sandbags in this style of training. When your session is done, the bag can be easily hung back up on the hook, and you can continue on striking it as usual. Other bags may not necessarily be equipped for this kind of alternative use.
Everlast has always been associated with excellent quality of their products, and their punching bags are no exception. An Everlast bag will last you a long time and will continue to provide for your strengthening as a fighter for years to come.