https://opiniuj.pl/sindbad.html - I'd read these reviews carefully before considering them.
Generally speaking, they have quite a poor reputation in PL for their attitude towards customers and the conditions on board. The drivers are poor (often sourced from places like Uzbekistan!), and the conditions on board are really quite awful at times. For instance, there's a strong culture of passengers taking smelly food on board (hard boiled eggs, dry sausages, etc), while the buses are often completely packed. Then there's a strong tendency of people to take their shoes off and sit with their feet on the seats, especially older travellers. Legroom is also an issue, while a friend says that people tend to just recline the seats as far back as they'll go without considering the person behind them.
They're used more by Ukrainians than Poles these days, but unfortunately, Ukrainian levels of personal culture on public transport are also very low. There's also a problem with general alcohol consumption on board, which makes for a pretty miserable experience. Apparently vaping is also widely tolerated by drivers, which just makes things even worse.
Definitely, don't rely on them for advertised connections. The drivers are notorious for not providing correct information, and there are frequent complaints about people being left behind because of driver error. Arrival times are also very unpredictable. From what I can see online, they've also developed a strong habit of leaving passengers in the worst places for hours waiting for connections, such as in closed bus stations or in the old customs terminal in Świecko, which is not a place that anyone would want to spend several hours.
