Switching across multiple desktops (ie spaces) is really annoying. In particular, you switch (Cmd-Tab) to the app you want, realise the window is on another desktop, then Ctrl-arrow-left/right to the desired desktop, but there the windows are ordered as they were when you left, and you have to Cmd-Tab to the desired app again.

Alternative: Cmd-Tab to desired app, arrow up (while holding Cmd) to see all windows of that app, then pick the desired one (with arrows or mouse).