I love my 6.4. Granted I havent' had it long, and already replaced the entire fuel system ($8,000 - that the dealer paid). However, since it has been a beast. I have researched as much as one possibly could. The best thing for them is sticking with 5,000 oil changes and change out fuel filters every 10,000. As soon as my extended warranty expires the DPF is gone. But knowing what I know now, I am planning on keeping a reserve of at least 4,000 for the next repair - the cab usually has to come off. I got a killer deal on mine, but if you have the extra money, I would try to go with a 15+ 6.7. Don't be fooled in thinking the cummins or duramax are any less problematic. There is a lot of negative reading on the web for a 6.4 - but it they sold a ton of them, and most of the people that take the time to post about it are ticked off - there are just as many good stories floating around. If you find a decent 6.4 (09/10) - here is what I suggest - take it to a reliable mechanic and have compression test run, check the oil pan for excessive fuel (some is going to be present), and most importantly have upper and lower fuel filters changed and checked for any metal.
I am waiting for my neighbor to finally decide to sell me his 02 7.3 with 35,000 miles. That is the dream set up.