Best part about R-Studio is its simplicity. Being able to navigate inputs as tabs, manage your environment, and constantly see what data frames you have helps users with coming back to projects and being able to pick up where they left off with little downtime. It's also nice to navigate packages while keeping your console and code blocks open. Obviously, its nice that its open source as well and that it can deal with very large data sets, but that is unique to R, not RStudio.
Honestly there isn't too much I dislike about the program. There are some shortfalls in the ease-of-use if you aren't familiar with R, but in that case you shouldn't be using such a program. The only reason I didn't rate it 10 is due to the fact I've found it prone to crashing.
This has helped me work with other colleagues in R, which has revolutionized how we deal with large datasets. Just working in R console seems daunting to most users, but when they look at the interface of RStudio, it is much more approachable and therefore more people will use it.