The ML moved well thanks to leasing, but they came back at lease end in droves. It really killed resale, and that means that an ML is a decent used buy. You can pick up a 2000/2001 in the very low $20K's with low miles and in good condition. The ML was improved greatly in 2000, but still lagged behind the RX in terms of quality.
If the new ML has improved quality, I guarantee that the CDI version will hold it's value the best of the models. It will be in low volume and have an appeal to MB enthusiasts.
Personally, I never really saw the appeal. They don't have much more room than a car, suck gas compared to the V-6 sedans (thanks to 1200 extra pounds) and don't drive as nice as the E-Class they're based on. But, people like the idea, and right now Lexus' volume seller is the RX330 and Mercedes has to have competition. The ML320CDI will be a super alternative to the RX400h Hybrid.