The Samsung Galaxy S8 comes with a Hybrid SIM option. This means that there is a single slot for both the memory card and the SIM cards. It's hybrid, because you can decide whether to use a memory card or replace the microSD card with a secondary SIM card. So the good news is that the phone supports a dual-SIM configuration, but I really don't like the Hybrid solution. This means that if I, for example, fly to another country and want to use two SIM cards, I need to get rid of the memory card.
This is a bad thing, because I want the memory card to be permanent on the device. I want to put it in an forget that it's there. I wil probably install many apps and store a lot of multimedia data (e.g. photos, videos) on that card, and I don't those to be unavailable and import/download them again when I need to use a second SIM card.
According to the official website, the Hybrid SIM vary by country, so you might not even have this option.Some say that it won't be available in the UK and US. If you need this feature, just make sure to contact the place where you plan to buy the Galaxy S8 and ask them if it support dual SIM. It seems to me that western mobile carriers prefer selling single-SIM phones. Some people might say that I need to thank Samsung for including this feature, but there are already phones out there that have a dedicated slot for the memory card, so why Samsung with its new flagship smartphone can offer this.
Having a dual-SIM capability is definitely an advantage compared to a singe SIM device. That being said, I personally see this offering as a disadvantage in terms of usability. I like having an option to use a second SIM card from another carrier, but hate that it needs to come instead of the expandable storage memory card. What Samsung calls a "Customizable storage", for me it's really not that of a great offer, especially for people which want to be more productive and efficient with their phones. I would have preferred a dedicate card slot.
From my understanding, phone manufacturer use the Hybrid SIM feature to make more space inside the device for other feature and make the phone slimmer. The memory card slot is a very important feature nowadays, especially in high-end devices like the Samsung Galaxy S8. If I had this phone I'm surely going to use it to take a lot of pictures and videos when traveling abroad. When it's released, I intend to buy it before I travel to Portugal and Japan, and I really feel sad that I will need to give up the memory card in favor of a secondary card which I intend to use for intentional calls. Just so you know, even the S7 and S7 edge have a Hybrid dual sim functionality.
I think that a high-end device like the Samsung Galaxy S8 should have a dedicate memory card slot. It's an advanced multimedia device that will surely be used by people who will take advantage of the expandable storage option.