I think McCarthy is overrated by Evertonians. Don't get me wrong, he is a good player, and quite easily our best midfielder (not difficult considering who he is up against), but he is nowhere near good enough to hold down a starting place at the likes of Chelsea or City. He has a brilliant engine on him but he isn't very creative at all, he seldom assists and he seldom scores goals. He is miles away from being as good as the likes of Fabregas etc.
Chelsea have surpassed the stage when they are paying ridiculous transfer fees for 'good' or average players (like Shaun Wright Phillips), and they now pick their players from the very top table. McCarthy isn't in that same bracket of player, and Chelsea are more likely to target the likes of Rakatic, or someone at the level of a Modric or a Zidane, they aren't going to be targeting James McCarthy.
McCarthy is at the right club for him at the moment. I think he is good enough to demand a place in a side like Everton or Spurs (who are supposed to be challenging for the Champions League). I'm not sure he would command a place at the likes of United, Liverpool or Arsenal though. There's no need for anyone to have a breakdown about this, it's just my opinion, but I don't think I am totally alone in holding such an opinion.