I keep telling other users this: if you play modern metal and you haven't tried using the Mark IV model, you're just plain wrong! (Kidding of course)
Really though, I used different 5150 variations for a while, but the Fit For An Autopsy crew convinced me to try a Mark series after hearing them talk so much about the Mark V, and seeing them live. The Mark series are harder to dial in at first, because they don't work like normal 3-band EQ amps, such as the 5150. It's a bit more complex, but it gives you the options that - in my opinion - made the 5150 obsolete for high gain stuff. It's so much easier to get rid of the fat, and get a tight, polished tone.
Read the Mark IV or Mark V manual to get an idea about how the EQ controls are different, but basically your 3-band is for the "feel" and the 5-band graphic EQ is for the "tone". Along with that, some controls are very dependent on each other (gain and treble for example). Anyways, it's all explained in the manual, and once you get how to dial it in, you really see how incredible this amp is. It's my main amp for high gain rhythm work, and even a good amount of lead stuff.
Clear, articulate, organic, aggressive, tight... it's everything I could never get the 5150/6505 to be. Give it a try!