Aggressiveness from steroids is psychosomatic (I'm a dual major in nutritional science and psychology). Research in human males has show zero correlation between androgen levels and agressiveness.
It should help with the other side effects some, but keep in mind it will reduce gains as well, and definately will reduce the fat burning effects of non-aromatising anabolic steroids.
Finasteride blocks conversion of test to DHT, right?
If that's the case, you would definitely have less of anything that DHT has an effect on, baldness being a key one
You'd also probably have decreased libido and erection power. Might affect gains to a small degree, but not much b/c DHT is more androgenic than anabolic
It should reduse acne, also. Azelaic acid which supposedly stops conversion to DHT is also used or indicated for acne, because DHT causes/aggravates acne.