How good is VBR encoding?

I currently have most of my MP3's in 192kbps encoding. I've heard good things about VBR and was wondering if I would be able to tell a quality difference if I re-encoded everything in 192kbps VBR. Would this mean that the least quality for the least complex sections would be 192, with the more comlex areas getting a higher bit rate? If this is true, then is it possible to set the limit for the max bitrate used in iTunes when encoding in VBR?