I haven't used kraken, but I can say that I would not qualify Coinbase as good... it as at best "usable".

Unacceptably poor customer service, significant markup when buying bitcoin, sometimes a week or more delay between purchase and fulfillment, arduous verification process for new customers, being arbitrarily (from the customer's perspective) flagged for a verification even though you already sent them a photo of your passport 3 months prior, banning customers for sending coins to certain addresses, and did I mention unacceptably terrible customer service? Sometimes just never even responding to a customer ever?

I can't say it's not usable... I have bought bitcoin from there and I did receive it, but the experience has been very far from good.