Kraken is useless if you want to day trade. Their interface is easy to understand; I will give them that. But the background doesn't work fast enough. If you're going to trade long term, the site might work perfectly for you. Fees are on the low side – so it is a great place to get some XRP, BTC, LTC or what-so-ever.
But, don't expect them to process orders fast like other exchanges. If the market is normal or calm – things go fairly well. But if it gets volatile in Bitcoins, Kraken will surely fail desperately concerning their order books and their goofy interface. You will place an order, and it will take minutes to get your order in – click submit button, and you will have no idea what is going on in the background for minutes. You will not see whether it got in, nor if got filled at all.
This usually takes minutes – then it goes back to the new order window with the enlightening message that your order might have failed. They do not even know if it failed or not. So you will have to go back and resubmit it since it never showed up in your “current order Window.”
This can go on and on and on – until, all of a sudden, you'll find out that you just sold three bitcoins instead of only one.