With stock exhaust, it isn't bad. It will help with some economy if you drive easy. I only disabled mine because I have a louder exhaust and it sounds like a jake brake when it kicks in. On flat ground running 65mph, I have gotten 23mpg with the MDS on during one trip. Same trip without it I can only get about 21mpg.
Don't be scared of MDS. It will save you gas. If you don't like it kicking in at low speeds, you can always put it in tow/haul mode. This disables it.
All 2006-up Hemi's in 1500 trucks have it except for the MegaCabs.

 

yea, i guess with my flowmaster, its alot more obvious when it kicks in. it sounds like **** when it goes back and fourth around town and i can def hear it go in and out on the highway.

 

I have an '07, and I don't know if the MDS is really doing anything or not. It is actually one of the reasons I purchased the '07. Knowing what I do now, I probably wouldn't have worried about it.

I am curious if it's really netting me better gas mileage. I drive almost all city.

joe

 

I have a tuner and I most definately will not diasble the MDS. I think those who do cannot grasp the time it takes to switch from 4 to 8 cylinders as a milliseconds. The HEMI will be able to transition from eight cylinders to four in 40 milliseconds (0.04 seconds). For those who don't understand this is 4 hundredths of a second to switch.

I don't care how big your ego is, you can't feel the difference in the shift time. However I will never claim to know what anyone else needs but think of the fuel saving difference between is about 2-4 mpg depending on your driving habits. This is why I will keep the MDS on.