Upsource team member here. Since Upsource indexes your code just like any JetBrains IDE does, it gives you code navigation, inspections, etc. in the browser, which means its "best features" don't require using it as an IDE plugin.

Pros: I'm obviously biased so I won't go into those.

Cons: it's not a lightweight tool that's happy with 1 gig of memory. Configuring and waiting for code indexes takes a bit of time, especially if you're trying it out on large legacy codebases. It also won't manage a Git repository for you.