Ben Feldman is perfectly cast in this
I would have say Cristin Milioti is as well in the subject line, but I know there are too many lost HIMYM fans watching this show who can't handle seeing her in another role.
If the angry HIMYM fans can accept that the show is over, and this is not a new retelling of How I Met Your Mother, I think this show will do well.
And I'm going to say all the characters are perfectly cast in it.