I find it a totally pointless, boring 1st season episode -- just an excuse to have them interact with someone from the 20th century (which is also just a money saver.) Also a little fan service in that a lot of folks wished they could see the future like the Christopher did.
Both Naked Time (which was supposed to lead into this episode) and "Tomorrow.." have virtually nothing to offer by using a potentially awesome event like time travel. "Assignment:.." also makes it seem so routine as to be yawn worthy. Send an entire ship and crew back just to observe the past? LOL.
Thank god "City.." was so well done.
Imagine 3 other alternate universes that are boring and much weaker than "Mirror.." to dilute the awesomeness of that classic.