'Time Squared' is a second season classic that utilised a nifty time travel premise at a time in the show's run where time travel hadn't become 'old hat' and where the novelty factor really acted in the episode's favour.
The discovery of a duplicate Enterprise shuttle containing a duplicate Picard is handled with aplomb, and as the episode unfolds things get more ominous (the duplicate Picard is from 6 hours in the future, where the Enterprise will seemingly be destroyed). There's a genuinely creepy, unnerving feel throughout, helped no douby by a memorably intense music score.
In short, it's an atmospheric and entertaining episode that keeps the viewer guessing right up to the climax. Unfortunately, the resolution itself is perhaps a little disappointing (why would flying into the anomaly get them out of it?), but this is apparently because the episode was rewritten at the last minute. Originally Q was going to have been behind the anomaly and the end of this episode would have led into a subsequent Q episode (possibly 'Q Who?'). Overall, a keeper though and well worth a watch.