It's worthy to buy on steam? Also, it is actually the best rpg to ever be released?
It's definitely one of the best. I would snatch the classic edition from GOG however, EE doesn't really add anything beside handful of bugs, progressive gay half-orcish romance and bunch of shitty written, annoying characters.
To be fair: The EE games were buggy after release, but they fixed them. Now they are playable without any problems. Also, most of the relevant and "big" mods have been ported to the EE.
That being said, you can easily get your own "EE" (and save some money) if you read and follow the modding guide at GOG.
If you are to lazy to mod or if you really NEED all your games on steam for whatever reason you can buy the EE games (BG1 is absolutely worth it too) on steam without worrying about bugs. I would wait for a better discount though, -40% is just not good enough.