The character himself is fine. In fact its quite refreshing to have a 'nice' boy on the show and i can overlook the accent thing as an American accent would only grate and at least they've learn't a little from the Vicki debacle. The problem for me is how weak the storyline introducing him has been. Michelle couldn't stand Peggy and had kept the secret for over 20 years, why would she write her a letter spilling everything now?. Ian and Sharon just shrugged it off like it was no big deal purely because with Michelle and the other Fowlers not there its made no impact and i'm actually on their side throughout this whole 'you should tell him who you are, you'll be the best daddy he could ever want' scenario i'm not on Grant's which i get the impression i'm supposed to be. Mark has a Dad, he's not missed out on anything by all accounts and he's turned out a lovely lad for it. Grant's missed out on a son but at the end of the day Michelle wasn't likely to let him be a part of Mark's life anyway so it wouldn't make much difference.
Mark's storyline could easily have been written more organically with Mark visiting Martin for any number of logical reasons instead of all this contrived nonsense with Michelle and her stupid letter. It all seems to have been done so the producer can take credit for being being 'the one that finally brought that secret to light', the problem now being its been done so badly they might as well have not bothered playing with even more of the show's history just to get Ross Kemp back for a 3 week guest stint.
Mark's nice, he's good looking and i like the nod to him looking like his Uncle but he just seems out of place in the Square. He works fine as a guest character and could work as a regular character once Grant's gone again but this storyline has made his connection to the show seem a bit weak so i'm not sure what scope there really is for him (or Courtney for that matter) as regular characters once he's gone. They might work better as a Libby Fox style recurring characters.