Alpaca Arvostelua 

I initially alpaca.markets a try because they were the only broker offering what seemed to a decent REST API at the time.
For one, beware of the spread they offer on most securities, it's huge compared to other brokers. Still manageable, though.
Now, the really ugly part. Like others commenters, things went messy when I tried to widthraw. Their UI is plain broken, and they managed to loose track of my funds for a month and a half now, having sent them to an old closed account of mine.
Support is utterly useless, and don't event seem to understand how their own payment processor (CurrencyCloud) functions or how money transfer does work.
All they do is trying to buy time over and over...
Definitively not on my recommendation list.

Twice they have lost considerable amounts of my money transferred in and out of my account. In my opinion they are not fit to handle client money and should lose their licence

If you get past the UI bugs and manage a couple of profitable trades, then you'll hit a UX wall. First, the popup for withdrawal lists states that you need to contact support to withdraw the money. Then, the support will ask you to use the website to trigger the request, but due to a bug, you have to do some JavaScript console magic: they provide an 8-page document to explain how to enable the old UI (you will need some years of experience in web development to understand it). And after you finally fix their provided scripts you get a message that you will be contacted by support.
And then three weeks pass by and nothing happens.
I was writing them every day for two more weeks without a result. They are just asking me to be patient.

Everything except money withdrawal seems to work eventually, is this an accident or yet another shady service...