Communication with Karl was great. His father met me at the apartment and he was very kind and helpful to me. The apartment is quite large and fully equipped with all that you need for a very nice stay (*parking available too).
The bed was very comfortable, the kitchen was perfect, the shower was fabulous, and the balcony was a nice feature (though extremely tiny) :) The location was a fine alternative to staying in the downtown area... and it's in a hipster-type neighborhood with plenty of shops and restaurants, and there is a small food mart downstairs open til 10pm right below the flat.
I had a great experience and I wouldn't hesitate to recommend Karl's place to anyone wanting to experience *beautiful* Riga!