I am currently starting development on a FPS game. I haven't decided on a developement tool. I am really drawn to Unity 3D but not sure how good the software is compared to others (the game has to have smooth multiplayer). Does Unity have multiplayer support? I couldn't really tell from the website.
If you are at this junction for hobbyist programming or self-teaching, my advice is to simply dive in. You will learn a lot simply making any decision. In particular you will learn that no engine is a magic bullet for building your game. You will see useful things already built for you, and you will see shortcomings requiring raw development time for logic specific to your game.
If this is for a professional project or you are simply worried about investing time in a particularly poor fitting system by mistake, I think you can look at the adoption, community size, and multiple platforms of support offered by Unity and deduce that it's a mediocre choice at worst. Personally, I have been using Unity for about 6 months now and find the engine to be time saving and enjoyable, and the community to be active.
Unity does have multiplayer/networking support however I have not personally used it yet. From a few minutes of Googling, this PDF Unity Multiplayer Tutorial looks useful and up-to-date enough.