People might think it's a little early to call, but I believe that Sin Cara will be a failure in the WWE.
1. He doesn't have anyone to wrestle. Cruiser weights like Sin need other cruiser weights to put on great matches. That is what made the cruiser weight division in WCW so amazing. The guys didn't have to play the cat-and-mouse games that all high flyer's have to play today. Guys like Evan Borne and Kofi may be "high flyers", but they don't work the lucha style at all. Sin needs other people that can move at lighting speed in the ring to amaze the fans. The "big guys tries to get him but he escapes" match gets old very quickly. (I'll address the Rey Mysterio comparison in the end).
2. His look is terrible. The WWE thinks they struck gold with the money they will make off the amazing mask that he wears to the ring. I'll agree that it looks awesome, but it will keep people from ever relating to him during a match. You simply can't see any expression. Other high flyer's wore masks, but you could see their eyes, or at least their mouth. Sin's mask prevents anyone from relating to him. This aspect is incredibly important to a wrestler getting over whether he is a heel or face. With that mask people wont invest in him emotionally anymore than they would to a Power Ranger toy they see a kid playing with.
3. His style won't work. People keep saying he's botching moves. However, that is really just the style he works. You saw it a lot with WCW cruiser weights. These guys move so fast that if they mess something up they don't even slow down, they just instantly do another move. It was never a problem in WCW because the whole division worked like this. A guy like Sheamus certainly won't be reacting to a botched move this way during a match.
4. He doesn't speak. His only works for monster heels. Underdogs, and he will certainly be billed as one in every match, need to cut promo's. This isn't just a creative decision. From what I understand, Cara barely speaks any English. Again, this wouldn't be an issue if they actually had a cruiser weight division. Today's WWE is run on intense feuds, passionate interviews, and drama. A guy that just flips and flops in the ring wont stay impressive for long.
5. The Mysterio comparison. Sin won't be anywhere nearly as successful as Rey was. Rey didn't get his fame by playing the cat and mouse games he's played during his WWE career. He gained his fame through high flying matches that American wrestling fans had never been seen before. He had 20 different guys he could go out and have a different amazing match with every night. He would have been nothing if all he did in WCW was run in between the legs of bigger guys. WWE also had a fairly decent cruiser weight division when Mysterio signed with the company. Another important fact about Mysterio is that, while an exciting wrestler, he became a top guy do to outside forces. Rey was popular, but the death of Eddie is what shot him to the top of the WWE. Before his heartfelt speech and tributes to his dead friend, he was simply an upper midcarder. I don't ever see Sin connecting with the fans on an emotional level like that.
I think Sin Cara is the Take Michinoku of the 21st century. Taka was an amazing high flyer who the WWE brought in to start a light heavyweight division. Unfortunately, he was basically the only one they brought in. He didn't speak English and after a few high flying matches, no one cared about him anymore. I think Sin Cara will have the exact same results.