BeckettBrigade said:

It was absolutely the worst ending anyone could have imagined.  Why in the he** would Castle hook up with the ex-wife he never had a kind word for and constantly avoided?  It makes no sense except to keep Castle and Beckett apart.  It was forced and orchestrated and I hate what they ruined.  They both grew so much recently and now this stunt just ruins everything.  It was a total lack of creativity or respect to fan loyalty.  Now the show in my view is ruined.

I don't think it was forced or out of character, as much as it might seem that way.

During the episode, Castle was slowly coming in terms with the fact that Beckett had indeed chosen Demming over him and he was only gonna get hurt if he kept following her around like a puppy. But his feelings weren't gonna disappear anywhere and he needed a distraction, so he turned to the only option available – his ex-wife, who had been calling him non-stop the last couple of days. Castle doesn't love Gina or want to get back together with her, but now that Alexis and Martha are gone and Beckett is with Demming (or so he thinks), she's the best he can find for filling that empty place in his heart.