First, I enjoyed the episode. But there were some problems with logic that I had. It made no sense for these aliens to find a specific era in hostory where they could "hide" thier work behind an epidemic of some sort. They were aliens who could travel between worlds, through time for Chrissakes. They could go anywhere, anytime. They wouldn't have had to limit themselves to that one time and era.
It was pretty silly to have Samuel Clemens on the Enterprise, but I admit, it was fun and I enjoyed it.
Like anything in Trek, if you ask four fans, you'll get five opinions.