A link to a Sound On Sound review.
The SPL Vitalizer is a piece of pro audio equipment that at heart is an dynamic equalizer. Describing it makes it more complicated than it really is. In fact, it's fairly simple to use and it can definitely make most rock/pop type music sound better. As a practical matter the unit needs to be positioned at the listening position in order to make quick and repeatable adjustments. Not a problem in a recording studio, but could be in a home setup.
If you're an electric guitar player you are probably familiar with using the guitar volume and amplifier gain controls interacting for compression and overdrive effects. The Vitalizer can do that (and a little more) with a more precision.