I've used K6 as well. I like it bcoz it's code, but my (major) complaints are lack of auto-complete (intellisense) in IDE and debugging. Without those two, it's hard to do exploratory testing.
Regarding debugging, that's unfortunately unlikely to come any time soon... We'd need support for that in the JS runtime we're using (https://godoc.org/github.com/dop251/goja) and then we'd need to figure out how to expose in in k6, while dealing with potentially hundreds of concurrent JS runtimes (VUs). Not impossible, just unlikely to land anytime soon...