Pine and Quinto did a decent job with the characters. The comparisons between the new and old Kirk and Spock aren't fair because they were trained differently. Pine doesn't have the extensive Shakespearean training that Shatner had, or the range of experience. Quinto doesn't have the same experience as Nimoy. The younger actors did a commendable job despite the differences.
The only problem I have with younger actors today is they don't seem to have the same vocal depth and projection as their predecessors. Also, younger leading man types today look physically smaller than the generations before.
It's not good or bad, it's just different.