I've been collecting hockey cards nearly my whole life.
There are two things that just bother me about the C55 set:
1) They were drawn up by an artist and appear too cartoonish for me.
2) If you collect rookies, then it is not that big of a set. The Vezina RC is probably the biggest reason why a lot of collectors get drawn to these cards. The Malone RC is the other big one, but the sad part is a majority of hockey fans don't even know who he was! It's an undervalued card, IMO.
Now the C56 set is one of my favorites. It is the first hockey set ever issued and is loaded with rookies of HOFer's, such as Cyclone Taylor, Art Ross (2x), Newsy Lalonde, Patrick brothers and more. Yes, they are more difficult to track down, especially in higher grade, but that is what separates them from the C55 cards. Furthermore, I think they have far better eye-appeal.