I've played and completed:

Red, Yellow (well, it was a friends copy), Silver, Crystal (on ROM, I was bored one weekend) and Sapphire.

I did have a play of Fire Red, had a nice squad of level 12 monsters before Brock, but my save magically disappeared thanks to this computer, and I never had the motivation to restart.

The games are fun, especially if you can battle against your friends, but all the other stuff (the countless pieces of merchandise, cartoon, etc) never really appealed to me.