I picked up one of these at my local McDonald's for $4.49.
Much of the sandwich's flavor comes from the mustard sauce which is yogurt based giving it a bit of a tang and incorporates Dijon mustard and horseradish which imparts a very solid mustard taste. I'm not sure it goes that well with the crispy chicken though, I think it would go better with beef or pork. There's a slight bit of sweetness to it.
Bacon wasn't crispy, but added a bit of smokiness. With competitors like Wendy's and Burger King trading up to thicker cut bacon, McDonald's really needs to follow suit. Their bacon doesn't lag in flavor so much as texture.
The white cheddar used is pasteurized process (i.e. made like American cheese). It was creamy, but with less flavor than American cheese and lacked any hint of distinctive notes of cheddar. If it wasn't marketed as white cheddar, I might guess it was white American cheese... It should really be the "Cheese Bacon Onion?"
Nutritional Info - Crispy Chicken Cheddar Bacon Onion Sandwich (227g)
Calories - 630 (from Fat - 260)
Fat - 29g (Saturated Fat - 9g)
Sodium - 1630mg
Carbs - 61g (Sugar - 13g)
Protein - 32g