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.
Unity3D is indeed a very popular platform and is becoming increasingly popular among indies and studios. Multiplayer network games though are not so prevalent currently. I believe this is mostly because it is difficult and requires a lot of development effort to get right. First there is all the socket programming on the client, managing buffers, threads etc. in Unity3D. Secondly you will need a server for routing, discovery, matchmaking etc. Thirdly on the client side, developers need to add handling for latency and use prediction techniques.
There are some cloud solutions which aim to resolve the first two problems i.e. client side networking and server side infrastructure. This does make it easy for developers to get started with their multiplayer games on Unity3D and dive straight into whatever business logic they want in their games.