I don't use RetroArch but the basic idea of it is to have uniform audio/video settings, controls, etc across multiple emulators. Most people like the shaders, auto-config controls (although this is also the thing I see ppl complain about), netplay and apparently reduced input latency. Some emu cores have improvements over the standalones while other cores are lacking features or worse than the standalone versions. It also runs on pretty much every platform out there. The other big complaint I see often is that the UI isn't intuitive.
I do however use RocketLauncher. One of the benefits of RL is not having to mess with command line options for your emulators. You point RL to your emulator, rom path, and set whichever options you want for the emulator and that's it. Then you just set your system in RFE to use rocketlauncher.conf as the launcher. Since the emulators are setup in RL you can have multiple frontends configured at the same time just by setting the frontend to call RL. I currently have RFE, LaunchBox, and Attract Mode all utilizing the same settings in RL. When I switched to RFE from HyperSpin I didn't have to bother with configuring launcher files or anything.
RL also allows you to set options per rom so games that require special configs aren't an issue. There's also the Fade (loading screen between selecting a game in the frontend and the emu loading), Pause (enables you to view maps, guides, media while playing and use/load save states), bezel and keymapper support. There's probably a bunch more stuff that I'm forgetting or don't use too.
However, with all those options the UI can be a bit intimidating to new users and some find it difficult to navigate or troubleshoot. Once you setup a few systems and learn the routine I don't think it's bad at all though, it takes me only a few minutes to setup new systems now or tweak things but there's definitely a learning curve. RL also only runs on Windows so if you're a Linux person it's a no-go. If you're only using a few systems, don't have the need for per rom configurations, and don't really want any of the other features like fades, bezels, pause, etc. then RL is probably not worth it for you. I love it personally, but you'll have to decide whether the time and effort to learn it is worth it for you or not. If you have any other questions about RL I didn't cover feel free to ask.
