I have not had any trouble using any version Vicki or Mike in Poser 5. Any model that uses features that are only available in Poser 5 will have backward compatability problems. To my best understanding the Millenium figures do not use any such features. By the same token almost every Poser model currently available has "backward compatability problems" with Poser 3 and even more "problems" with Poser 1 & 2. Since DAZ is developing its own software I can quite understand why they would have no reason to encourage the use of Poser 5.