This is among the first TNG episodes I ever saw and so I have a soft spot in my heart for it. That said, it's on on the same level as the previous two season-ending cliffhangers ("Best of Both Worlds" and "Redemption"). While not the first time travel episode in TNG, I believe it is the first in which the TNG had to travel back in time "to save history."
Every moment the camera is on Brent Spiner is worth watching in this episode. The writers certainly had fun imagining how Data would adapt to 19th century San Francisco and it shows.
On the minus side, the parts with the rest of the crew are pretty unremarkable setup for the season 6 opener.
Fun to watch, but unless you love time travel episodes you won't find this to be an earthshattering one.