I have the Motif ES Rikki. I did have some issues with it and OMB on a few of the Yamaha styles, but I am now at version 8 with a faster computer and I haven't noticed any problems.
I will tell you that some of the Motif's GM voices are pretty good, while others are not so good. I considered getting the XG plug in as I still have one slot available, but I am not certain whether it would sound that much better or even any better than the SYXG50 XG software module I have.
Actually, although it's only 32 mb, I think the freewsare soundfont - another XG module, sounds at least as good and maybe better than the SYXG50.
AJ
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AJ