Depends a bit on style. But, I will say this - 90%+ of my beers that turn out very well, taste quite good from the fermenter. They have all the flavor I would expect, none of the flavors I would not expect. They are flat, and a bit warm - but otherwise - they taste as they "should."
Bigger beers, beers with spices, oak, additions, etc...... those can seem harsh and over the top early on - and aging lets things mellow. But, your "everyday" basic styles will be pretty close if everything is as it should be.
The presence of "off flavors" can age out of a beer - depending on the off flavor.
The descriptor of "rubber/plastic" does sound like chlorine/chloramine as mentioned earlier. What did you use for water? Tap water with chlorine/chloramine as brew water can cause this. Using bleach as sanitizer can leave chlorine behind. Even rinsing/sanitizing with chlorinated water can potentially be an issue - although, not as much as the other possibilities.