It all depends on the quality of the OP song TBH cause it can really boost your enjoyment of the show seeing as you have to listen to the fucker like 12+ times anyway its good if it rocks!!
For example look at the Kimi Ga Nozomu Eien OP:
Precious Memories is one of my favourite songs of all time and without this awesome tuneage I think the OP wouldve sucked hard. (It gets me totally ready for the show)
Still having a killer OP doesnt necessarily guarantee success, Ive never been more disappointed in an Anime than Hajime No Ippo: New Challenger! (The OP song totally rocks but oh my my my... Its one bad show)
I was like "HOLY SHIT... Its on now" So psyched about it!
Then I watched it and a wave of disappointment washed over me, May as well call the fucker Hajime No Takamura! :(