I've had two Rams with diesels, and now the 1500/5.7. The 95 2500 with 5.9 Cummins and a 5-speed was a good truck, and even after I hot rodded it (370 injectors, completely recalibrated P7100, larger turbo and a 4" exhaust), it would get ~20 mpg empty on the interstate if I kept the speed to around 70 (3.54 gears). I'd still have it, if it wasn't rusting away, thanks to all the winter salt.
My 2011 3500 SRW with the 6.7 Cummins and 4.10 gears was a much more refined truck, and for the 4 years it was deleted, it was superb. Deleting didn't net the fantastic mileage gains I'd frequently read, but driveability was much improved. As soon as inspections here began looking for diesel emissions equipment (installed & operational), I returned it to stock and drove it for another year. Highway mileage was 16 on a good day. Deleted, it was around 18, both unloaded, not towing. I will say, I towed 16,000# (JD 1010 with a huge Ford backhoe attached, while it was deleted but no additional power to avoid shredding the transmission) for several hundred miles, and that truck handled that load like a pro.
The new diesels do get decent mileage. From what I've read on the TDR, around 20 highway unloaded, but like many have stated, the emissions equipment is complex and has way too many potential failure points, not to mention the eventual need to replace the DPF for $2k+ as the truck ages. Had I a need to tow heavy periodically, I would probably have another diesel.
I thought long and hard about getting a 2500 with the 6.4. However the 3500# enclosed cargo trailer I tow periodically is no issue for my 1500, and the 1/2 ton is way more comfortable. Most often, the only thing this truck hauls around is my butt, and I really didn't need another HD truck to do that. The 1500 has gotten a high of 21+ mpg on the interstate, and thats with 3.92 gears. Towing the trailer it drops to ~ 14, but the trailer is 6' + tall and wind resistance takes its toll. No regrets tho.