I have the Diablo Predator. It works well once some of the paramters are changed towards better towing. If I did it over, I would go with 5-star since their tunes are made with towing in mind.
I'm running an 87-performance tune. I was actually surprised how much improvement this made. Enough so that running the higher tunes on higher octanes made much smaller gains beyond the 87-tune. It is on the edge of pinging though. I actually have to be picky about where I get my gas, because some brands will make it ping with that tune (Chevron/Texaco, Safeway & Fred Meyer are my safe brands.... Seems hit and miss with shell or 76.... I won't gamble with the no-name stuff anymore). I do carry the programmer just in case I get a really bad tank and have to go back to stock temporarily.
Engine is holding up great. Beside the programmer, I also have the full Banks PowerPack and AFE Stage-2 intake. I run the van hard and fast, with most of the miles towing. I'll hit 356,000 miles later today.
The extra tuning does make more heat. I put a larger transmission cooler in (Tru-Cool Max 40k) in and both a trans and diff temp gauge. If I'm towing really heavy, its my diff temp I have to keep an eye on (yes, I am running syn and I have a high-capacity finned cover).
The main issue directly related to running the tuner is spark-plug life. Double-platinums are worn way down by 60,000 miles. Last time around I tried AutoLite Irridium plugs, and they look great still! I should be back on a 100k tune-up cycle with the Irridiums.