There are many reasons to like Coherence, below are five of my favourite reasons why I like this product. I have taken away some of the most obvious ones, like it's performance & scalability, and will try to point out a few ones that might be missed at the first glance.
1. It's a Very Cool Product
The last time I ever saw a product that made me raise my eyebrows was when I first saw TopLink, which I guess was sometime back in 2002. At that time I remember that it was a struggle to efficiently map Java objects to relational data. Using pure JDBC calls and populate the objects from the result sets were the common way of doing this back then. It often took efforts & resources to achieve this somewhat effective. TopLink solved that problem so nicely, and all of a sudden the problem no longer existed. Today I feel that with the emerge of SOA applications, we see more and more problems related to performance & scalability and I firmly believe that Coherence will be one product that will help greatly here. (Sorry, I could not resist mentioning performance & scalability...)
2. It's Designed to Handle Failure
At any time any given Coherence node can be taken away from the cluster, and the cluster will still continue to work without interruption. The idea is not to protect individual nodes from failing, but to keep the cluster as a whole alive. In order to do so, Coherence has at any given point in time a pre-defined backup plan on what to do with the data in case of failure of an individual cache node and how to distribute the load among the other cluster members. This is a nice one. Most applications works the other way, they are designed to stay up and does everything to prevent them from dying, even if this means dropping client connections, for example. The clients were lost, but the process managed to stay up. Reminds me of the saying "Won the battle, but lost the war...". I think this approach is one that more applications should take; don't worry if you loose an individual component of the application as long as the application as a whole stays up.
3. It's Easy to Install
It's a small zip archive, ~8M. You just download & unzip, and the installation is finished and you are ready to run. No installers, no product registry, nothing. I really prefer these types of installations to others. It's simple, it's easy and it works.
4. It's Configurable
...and it has reasonable default values. As far as I can recall, I have never came across an application that has such a vast amount options of options in order to alter the configuration to get it to work as you want to. Sure, most applications have the option to alter some of the behaviour with Java and/or XML configurable parameters in order to tweak it, but so far I have not seen any application that comes even close to Coherence on the amount of options you have. Oh, I almost forgot to mention that you could for the options use either Java or XML; you are not restricted to only one of them, which unfortunately is quite common. Did I mention that I also think that most default values make sense, which is unfortunately not so common?
5. It's Extensible & Modular
Suppose that you don't like some parts of how Coherence works. For example, you don't think that none of the default caching schemas that Coherence provides really provides what you want. Well no problem, Coherence provides you with a vast set of Interfaces that you can use in order to build you own implementation that behaves the way you want. Some examples are CacheMap, CacheStore, CacheLoader, and AccessController etc, just to mention a few. This is something that I'd like to see more of in other applications as well.