TLDR: A fairly average movie, but the utterly ridiculous plot premiss knocks the overall down from average to below-average.
The whole premises of the conflict between the two main characters, Sam(antha) and Matt, made little to no sense. None of the following gives anything away that isn't seen in the trailer.
Sam is extremely mad at Matt for convincing her father to divorce her mother. Generally something that could cause some anger, sure, but her father left her mother because he's gay and came out. Sam says she's happy for her father living his truth, and yet for some inexplicable reason her anger with Matt is so great that she's willing to make an outrageous bet that if he loses means he can't be in the wedding (he's the best man).
Now clearly this makes perfect sense because...wait, what??? It's not her wedding. It's not Matt's wedding. It's her father's wedding! So if she wins a bet, that neither groom is even aware of let alone involved with, then somehow she gets to decide that the best man in a wedding that isn't hers suddenly can't come anymore? What???
Plot premiss aside, the leads do the best they can with the script and screenplay they were given and they have decent chemistry. They start out a bit stiff, but the pair gets better as the movie goes on. I feel like Anne Patterson, who played Milly, actually had the best performance of the movie though. She played the shy but "adorkable" role well, and I feel like her and her love interest actually had more chemistry than the leads. The rest of the movie is pretty typical for a TV Rom-Com. If you like them great, if you don't then not great.