Is this her first relationship?
She's probably just excited about having this new guy she's crazy about, and wants to share her excitement with you. To her, right now, she probably sees this guy as amazing and perfect and everything she's ever wanted. It's going to be hard for her to see why you wouldn't see him the same way, and as her good friend, she probably seeks your approval in some way. When a person is overwhelmed with fresh feelings of love, it often can cloud their vision of how others might feel when they let their romantic feelings gush out into everyday conversation all the time.
That being said, it doesn't make it any less annoying when you have to be on the receiving end. Talk to your friend and tell her how happy you are that she's happy, and that you think he seems like a nice guy (though if he doesn't, don't lie). Spend some time letting her vent her happiness to you. Once she says what she needs to say, she's more likely to ease up afterward.
If you do that and she still wants to do nothing but talk about her relationship, be honest with her. Again express that you are very happy for her, but that sometimes you'd like to talk about something else and that it's a bit overwhelming for you. Don't be afraid to redirect conversations in another direction if you need to.