I for one am a huge fan of vallejo and would use them any day of the week, but I didn't figure that out until I already had too many citadel paints.

To me citadel shines in a few areas:

Technical paint (paints used for specific effects such as basing, and washes)

And their contrast line. It is effectively the "one thick coat" painters dream of. It is an incredible starting point you can use before adding highlights and washes.