I am planning to visit Stockholm and then go to Gdansk, Poland. Is the ferry worth it, or am I better off flying? Trip is planned for late April
If you haven't done an overnight ferry in Europe before I think it is worth the experience. Especially if you reserve a nice big comfortable cabin. If you have done that before or are on a tighter budget, it might make more since to fly:
There's an airline called Wizz with very cheap fares, but like other ultra-low-cost airlines you need to pay ahead online for not only checked bags, but carry-on bags too. Wizz also uses an airport about 1:30 bus ride from Stockholm instead of the main airport in Arlanda, which is closer to the center of town. SAS flies to Gdansk as well, with higher fares, but they fly from Arlanda, include one checked bag and the seats are the typical 31" pitch instead of the 29" on Wizz.
If you have the time, go by ferry. I would. The Stockholm-Gdansk route is with Pol Ferry, a very convenient connection. I've the taken the ferry but I am sure the comfort is much improved now.