The Dreamcast controller is similar to the XBOX controller, the only difference is XBOX has more buttons.  The triggers are pretty much the same, the analog stick is above the d-pad, so if you added Select, Black, and White buttons, and another analog stick, it'd be exactly the same as XBOX.  However, the lack of these in Dreamcast's severely limits gaming possibilities.  For most PS2 games, the right analog stick allows players to rotate the camera around their character.  Without that, if camera controls are needed in a dreamcast game the L and R triggers normally are assigned for that.  This limits the options for useable buttons.  Also, that right analog stick is sorely missed in first person shooter games, which are rare on the Dreamcast to begin with.  The games that are available invariably use the A, B, X, Y buttons for directional movement, and the analog stick for aiming.  So, when compared to Halo, which makes use of those buttons on the xbox controller for reload, jump, switching weapons, and melee attacks, they are incredibly hampered by this.