Oracle VM is open source so it is free and readily available for downloading.
It saves a lot of money instead of purchasing multiple servers, and can allow you to load more than one operating system at a time.
Use of Oracle VM and installation is quite simple.
It has high security and works well with a wide range of hardware.
Room for Improvement:
Oracle VM has some of its versions supporting only English language. So you can think of what happens to non-English speakers. Using it on Web services APIs -- standard access is enabled by default, which is a security flaw.Although open source, downloading some of the applications is very tedious because they are too large.
Disclosure: I am a real user, and this review is based on my own experience and opinions.
Jan 22 2013