Interface is very intuitive, apart from the strange Queue. As it takes just seconds to add titles to a track, I'm not sure why it needs a render queue at all and that mux operations are not just performed when ready. Probably goes back to a time of weaker and single processor computer. Worth considering retiring queue.
What's especially awesome is that if your subtitles have an issue, it's very easy to preview the rendered file and then re-edit with corrected subtitles without having to rebuild from the original media file. I.e. Subler is non-destructive, it's final videos as editable as text in a Word Processor. Keep in mind you do need a subtitle application and the only one which is any pleasure to use at all is the fairly expensive Chinese Subtitle Edit Pro available on Apple's App Store only.
Unlike Apple's software, the nitty gritty (which is important when encoding) is not hidden from you in Subler. But it's not cluttered either. Subler is one of the most impressive free applications I've ever used and much better than many expensive AV tools. Real professional software.