I think they're terrible looking in design and no where near as good as it all once was. Nostalgia? I'm not sure but for people who do disagree and do say it's nostalgia, I only wanna hear opinions from people who grew up with the first gen and played it religiously like we all did back in the day.
IMO, Gen I, II, IV and VI have amazing Pokémon, Gen V has very good Pokémon, and Gen III has the highest concentration of terrible Pokémon (still a few good ones though). Gen III was the low point in the series for me, I've never seen any sort of decline over time despite how attached I was to the first two gens as a kid.