Depends if we're talking about being disappointed at the time or disappointed when we play the games ten / fifteen years later...
I bought a CD full of Spectrum games and emulators for my Amiga, hoping to relive those halcyon days when I'd play Elite, Stormbringer and Starglider for hours on end. I had a very rude shock when I sat down and played those speccy games again and remembered just why I wanted an Amiga so badly in 1990. Those old 8 bit games came with 8 colours - all garish - and a mind numbing plethora of tinny and slightly out of key bleeps courtesy of the emulated AY chip. At least they didn't manage to emulate the fifteen minute wait for games to load from tape though!
If you're in any doubt as to the greatness of old Amiga games, just spend half an hour playing Turrican II and your cynicism will disappear... If there's a better shoot em up out there I've yet to play it! I do still keep coming back to Batman the Movie, Lotus III, Civilization, Warlords, Doom I & II etc.
Being taken in by hype is nothing new though. Remember back in 1989 when Psygnosis released Shadow of the Beast? A truly beautiful game that was distinctly mediocre in terms of gameplay. Xenon II? Great graphics and lovely sound, but the scrolling was jerky and I couldn't be arsed to try and get past level 4.
The biggest offender though has to be US Gold with Outrun. How could they take that beautiful Sega coin-op and screw it up so badly???