You don't actually pay for the emulator, but yes for the games. You can rent Sega games for several systems, including Saturn, that you download to play. I can't check it right now, but i remember each game is around 200 Yen a month.
There is one small demo you can try, from Shining Force III (a 100Mb download, on the links at the bottom, the Heavy Power Check) . Altough the emulator is fast, you still need lots of processor power, at least 1.2Ghz, with heavier 3D games coming close to 2Ghz and up. The main feature, however, is sweet, sweet, actual _sound_.