Yes, Stephen Colbert will fill his shoes perfectly. Not only is Stephen Colbert a good entertainer, but is experienced in the talk-show role. He also is similar to David Letterman in that he takes everything with a grain of salt, and treats his guests with respect. I believe that he will do very well on the show.
In my opinion, no one is irreplaceable. I believe David Letterman is/was fantastic and I feel it will take quite some time but that EVENTUALLY Stephen Colbert will maybe not fill Dave's shoes but definitely create his own pair of shoes that are equally successful. After all, he already has a good following.
Stephen Colbert I feel will do a good job replacing David Letterman. I know it is a tough spot to fill but Colbert had his own talk show as well so he he his experience with television as well. I think with his experience and popularity, he will do a successful job.
Through the years, David Letterman has become a household name and one that will not easily (and probably not successfully either) be replaced. Stephen Colbert may be able to take the place for some, but for true David Letterman fans, there will be no replacement that could entertain like he does.