The fundamental problem with putting a rollerball in a Sierra style pen is that rollerball ink is thin enough that it will dry out if not capped.
Personally, I don't like the easyflow 9000. I have bought several of them and never liked them. Your mileage may vary though, I have not heard of anyone else that does not like them.
My preference is the private reserve gel ink that Exotics and IPD sell. I have had several people ask me how I keep the rollerball from drying out when I have let them use those cartridges.