By way of a quick intro, I'm a newbie here (first post - be gentle!) and a current C63 owner.
I like the CLA - it's good looking, it's more spacious than the C63 inside and I'm told it's quieter.
That's before you get on to the subject of the fuel consumption, which has definitely got my attention. When you drive 20k miles a year and fuel costs around $8+ a gallon (I'm from the UK, but have adjusted the measurements and pricing to the US equivalent), it quickly gets expensive. I'm getting around 15mpg (again US figures) on mainly motorway driving, which equates to around $10,000 a year in fuel.
That said, I still wish they had put a bigger engine in the CLA45. Several of the US tests have put the real-life 0-60 time of the C63 at 3.9s, with 30-70 coming up in just 3.5s. I seriously think the CLA45 is going to struggle to meet these figures (but would be glad to be proven wrong).