Half of a console is in its games. Its that simple. There is one thing that Sega consoles had that others didn't: Sega games. If you love sega games, then these consoles are for you. Virtua Fighter, Sega Rally, Daytona, Virtual On, Shining Force, Panzer Dragoon, golden axe. These are the reasons to own a sega console. If these games arent to your taste, then dreamcast and saturn may not be where you should go.
What people have said earlier is true: There are numerous emulators for these two systems that work very well. There is also a significant number of sequels and ports of these classic sega games that have been made for other consoles now taht sega went multi-platform.
The dreamcast has an amazing one-up over all other consoles though. It has an amazing homebrew community that makes games for it regularly.
The second half of a console is a mix of its hardware quality and its controller. The Saturn is honestly one of the most robust consoles ever built. Another bennefit is that it has one of the most popular old school controllers ever built. Shmup and fighter fans love it and buy usb versions of it and converters for modern consoles. The saturn had 2 controller designs. I think both of them are excelent, and own several of each.
The dreamcast is less reliable than the saturn. I love the thing, but they do have a black mark in retro game shops because they often need repair unless treated well. The controller is also another complicated point. I really like it, but it is decidedly less popular in the video game world. This is for both its lack of buttons compared to competing consoles, as well as its more rigid design that demands a more specific hand position. Its definately a matter of personal prefrence.