It doesn't work very well at all. There's two problems.

1) There's no bind you could hold to auto-target the closest thing to the mouse cursor.

2) In order to successfully cast a spell on something (or shoot them?), I think you have to click on their tile, not the bot? I have often felt I couldn't click on a bot at all because of the walls that were in the way. It at least can't be done on the visible part of the bot. If you hover over a bot and it shows its name, this should be enough to enable targeting it as well.

I think I'll list this as important since it's a basic part of the gameplay loop. It's even worse since you can't pause the game to issue a command. The one argument against this could be that it's meant to be difficult to target fast-moving things, but that still leaves us with point number 2.