tl;dr - Great atmosphere, friendly staff, fantastic DJs, and don't forget to try the cocktails (or…read morethe coffee, if you're not into drinking)!
There's something about the ambience in here that I really love, and so I can easily say it's my favorite bar in Helsinki. This place is the site of the former canteen for the harbour workers, hence its name, and the interior decor is very representative of its history. Across the ceiling are numerous flags from different countries, and along the walls you'll see some photos of the old canteen. The furniture is classy, old-fashioned and gives the bar an incredibly cozy feeling, more like hanging out at home than in a public venue. The bar counter itself is part of a boat hull, which is also really cool.
The cocktails/mixed drinks are all great, and they've got a couple of local specialities that can't be missed, such as the Bus Island Iced Tea, a Jätkäsaari spinoff of a Long Island Iced Tea. The beer selection is pretty standard, so if you're not into the fancier drinks, you're covered. The coffee and pastries have also been good from my experience, as well as the selection of tea (although you're paying a bit much for a cup of hot water).
The music selection seems to rotate every so often - for the first couple times that I came here, it was mostly classic Finnish rock. Another few times, it was Latino music. The live DJs are both wonderful, especially the one who works on Saturdays - he was making up short poems as a form of introduction to the upcoming songs, as well as occasionally whipping out a small flute and playing it! Very spunky guy, and fun to listen to.
The staff are also fantastic - I didn't sit at the bar to chat with them, but they are definitely friendly and they like to converse with their customers. I think there is a large group of locals who come in as regulars here, and they've gotten to know the bartenders well. There's always a swarming crowd in the evenings on weekends at the bar, so be warned - get your seats early! It's much quieter on weekdays, but it's certainly not empty.