It reportedly offers better performance that regular VM and also has the option of only using File Mapping and not the part of VM that writes information to you hard drive. There are certainly situations where I can see that Ram Doubler would be attractive. It does have some nice features. However I don't think everyone should blindly go out and spend the fifty or more dollars for what it offers. I have listed the reasons below.
Real memory is always better.
The only advantage that VM offers over real memory is that certain applications may launch faster. Personally I am willing to wait a few extra seconds for an application to launch if it will be faster once it is running. The wait is well worth it. Although a single application may launch faster, VM increases the time it takes to switch between applications so its still a trade off. Ram Doubler will more than likely need to be updated every time I update my system software.