I was disappointed in Yellow when I got a certificate, I didn't know what I would get but I had hoped for something more.

I got my certificate in Red version, i too was disappointed, mostly because i didn't have the printer, i think an unlimited supply of Rare Candy's should have been the reward because most of the Pokemon you have are lvl30 an under, and having all the Pokemon in the game, what else could you possibly need apart from the candys? free items when you go to the shop?? maybe?

Another good possibility is that once you catch all 150 you get Mew (he was 151 i think, which is weird considering he was around before Mewtwo) or a secret Elite 4 or boss battle only unlockable when you have all 150.

Then you have officially used an action replay or spent thousands of dollars of going to a pokemon event just to get one pokemon and have played for 200 + hrs.

oh yeah.
you beat the game completely

i don't think that there is a party

my friend did it. but he didn't mention a party (in diamond) 

It's mostly bragging rights. The only pokemon I ever owned was blue way back when. I got pretty close, but was mostly just trying to beat the game.

I think, after you catch 'em all in every single Pokémon game, you look back and say "I've wasted my youth". I'm not positive, though.

@terran77 yeah, is just for bragging...but you can yous brag for a certificate and the star...which is kind of lame accomplish
Well, that was, now i've got to caught a couple of pokemon, i've on 383 so just 110 to go
lol

I don't really remember in the old ones.

It's just bragging rights. I have all 493 of them. Nothing special happens. I'm waiting for next version come out, which are probably remakes of G/S.

Red and Blue DO NOT use the Game Boy Printer. Only Yellow, Gold, Silver and Crystal use it. I have it in my bedroom, the printer that is.

I think you can print a certificate in Gold/Silver as well. I never really cared to catch all of the pokemon in any of the games after that. I don't really know if I really have bragging rights for taking over 300 hours to obtain all 251 pokemon. Should a person really be proud of themselves.