I often hear native speakers say "it is impressive", I think they want to say it is something which leaves deep impression on them, but is it a commendatory or derogatory or neutral term? Is it true different people have different views of this word?
The thing to remember about "impressive" that will help you understand native speakers is that "impressive" is the opposite of "ordinary" or "mediocre". Therefore, saying something is impressive is like saying something is extraordinary - surprising, unusual, or remarkable.
This makes the word flexible enough to use in positive or negative contexts. We would expect an "impressive performance" to mean they did much better than expected. However, we could also see someone have an "impressively poor performance", which means the opposite.
You should treat "Impressive" as a positive word, but be aware of the context.