I have played the games since the beginning, so I guess that by Pokemon fan logic, my opinions are worth more than the people who haven't played the games from all generations or in the right order. In all seriousness though, here are my thoughts.
If we look at the games at the time when they were new, I'd say they were good. I had a lot of fun with them and spent a lot of time playing them. They were also quite unique at that time (I suppose), what other RPG let you chose between 151 different characters to use in battles? Not many, if any at all.
However, when compared to the newer generations, the are in all honesty really bad. They lack a lot of things that the newer games has, which makes the first gen games less fun than the games from newer generations. Only being able to choose from 151 Pokemon drags them down a lot since there isn't as much variety as in newer generations. The lack of something to do after beating the E4 also limits them a lot, the only things you can do after the E4 are exploring the Cerulean Cave (as well as other places if you didn't explore them before the E4) and complete the Pokedex (which goes relatively fast despite the lack of training possibilities as there are only 151 Pokemon to catch/evolve). Not being able to breed in the games also makes it hard to get "exclusive" Pokemon such as starters and fossils, or even the base forms of some Pokemon. The lack of a story makes them pretty mediocre as well, though it doesn't really matter since Pokemon has never been a series with focus on the story. Compared to the newer generations, the music and graphics in them are really bad as well. Lastly, Kanto is also a really small and boring region without the Sevii Islands to back it up.
I remember that I tried to play Red in 2006, but I just couldn't do it. Even compared to the third gen games that were the newest then, the game felt incredibly underwhelming and boring to play. I haven't touched a first gen game since then. Many people talk about "nostalgia", but that doesn't effect me at all, I don't see the reason to play games for nostalgia rather than for fun. The feelings I had when playing R/B/Y as a kid can never be brought back, so why even try? I don't see the point in it. Even with nostalgia factored in, the first gen games still suck compared to the newer generations. I'd rather play the newer games that are actually good and more importantly, fun to play, rather than a boring and unplayable first gen game. And if I want to go back to Kanto, I'd much rather play FR/LG instead.
To put it simply, I think the games were good back then, but they just fail to live up to the modern standards of the series.