The show definitely became better after Misha Collins appeared on it but to be objective the writing and the plot were way better in season 4 and on.So Misha was one of the reasons of the show becoming better but not the sole one!

I would totally have to agree with you this show the whole plot is better since misha collins has arrived on it and another reason is that we found out that jensen ackles is michaels vessel to save the world. Its good because of misha and jensen but mainly misha

def better with Misha he's always been my fave character

Worse not because Misha was casted but in my personal opinion i found the show alot better before season 4 maybe 3 i was an avid fan but now i find that it's same ol same ol. I mean i think Cas is adorable and has freshened the show up a bit but i did prefer it before season 4 so technically before Misha was casted:)

While I think the show was always great I think the addition of Misha and the angel/heaven/free will/destiny storyline that came along with him just increased it for me. So I think that while the show was awesome before, since S4 it has just kept getting better to me, so I would have to say that it has gotten better since Misha joined the cast.