I remember my initial reaction was "Why am I playing MGS2 in MGS1's world?" It was a bizarre.
Also, seeing Mario and Yoshi dolls in the lab was immersion breaking for me for some reason not to mention the really long scenes coupled with not reusing original OSTs and beats made it unplayable for me.
I might have been hard on it back then, I might try it again though.
MGS1 shoehorned in Policenauts footage though, IIRC (although that was part of Otacon's rambling, and nobody knew what Policenauts was).