Although I joked with my family while watching that Hardy didn't much resemble Stewart (who does look like 20something Patrick Stewart? 70something Patrick Stewart. The poor guy was born old, I think), I was able to suspend my disbelief and enjoy Nemesis. Likewise Nimoy and Quinto's Spocks, despite one having earlobes and the other not (and FWIW, I'm almost certain they built up Nimoy's nose in the meeting-of-the-Spocks at the end of STXI, so he would closer resemble Quinto)
Slightly off-topic, I'm curious how X-Men: First Class
will handle Ian McKellon and Matthew Fassbender in the same movie as Magneto, when they look and sound absolutely nothing alike.