RiMac 5k audio speakers

They're definitely in the range of $100 computer speakers in terms of overall frequency response and volume. Distortion is low, lower than previous iMacs I've heard over the years. Accuracy and imaging are weak, there's an upper-bass emphasis and the sound comes out of holes in the bottom so it's bouncing around all over the place.

It really depends on your needs, best bet is to find any iMac somewhere besides a store and see how it sounds, there's no breakthrough here with the 5k iMac

Whether they're good enough or not is hard for anyone else but you to answer, but if audio is somewhat of a priority (i.e you might do a little equalizing, compression or mixing) I'd get a couple of flat sounding speakers like the Yamaha HS-series. You can find the older HS-50's quite cheap on Ebay. The iMac's speakers are not accurate, so what you're hearing while working with the audio with the internal speakers is going to be very different from when it's played back on other audio systems.