I'm a white American and my fiance was born in Pakistan, moved to the US with his family as a teenager. I studied Arabic in college and grad school, lived in the Middle East, and converted a little under two years ago shortly after I first met my fiance. I had been talking to my family for years about possibly converting, so by the time I did it they were not surprised or upset and I had had a lot of time to explain Islam to them so they would feel comfortable. They love my fiance and are very welcoming, have no issues whatsoever. I am close to my sis and usually our family goes to her house for Christian/American holidays like Thanksgiving and Christmas. She has been so great, she even stopped making a ham since I converted, which has always been part of my family's holiday menu along with turkey; even though I told her to go ahead and make her ham---I'm happy with my veggies and bread and some turkey! Most of my family members don't drink at all and never have, and we have a close-knit, modest, family-oriented culture so I haven't had any major transitional issues with them.
With the Fiance it's been a little trickier because he definitely comes from a very religious, traditional Pakistani background. But in the end his family has been great. They are very sweet, I've become pretty close to one of his sisters, and his brothers are all wonderful as well. I love his parents and don't think that treating them as my own will be a problem. They are definitely more high maintenance and expect more say/involvement in everything, which can be hard to reconcile, but I have a good temperament for negotiation and compromise so insha'allah this won't be a huge issue.
The wedding itself has been trickier to plan for. My family is very easy going and is fine with having whatever kind of ceremony we want. But in the interest of honoring them and making them feel comfortable I have tried to integrate some American elements into our wedding planning, and that's been somewhat uncomfortable for the Pakistani side as they have no exposure to American wedding traditions; I think it can be hard for them to accept things that are not cut-and-dry halal or haram (like an exchange of rings, with the understanding that it's not an Islamic requirement) or cut-and-dry Pakistani. For example, we're planning on holding the reception and dinner in an American restaurant. The wedding will be dry (no alcohol), and the restaurant will be using all halal meat and will incorporate some Middle Eastern and Pakistani foods into the menu (biryani is one of our entree choices.) But I think they're not used to the concept of having a mixed menu, are afraid that the extended family will complain, etc., etc. So it's a bit of a headache. It has been a bit disappointing to plan something I was at first very excited about, and to find that there is a lot of nitpicking---spoils the happiness a little bit. :(
I think mixed weddings/marriages can go either way. It just depends on the 'personality' of each family. In our case I think my side has been somewhat easier to negotiate than my fiance's, but some couples have no problems, have problems for both sides, or have problems from the non-pakistani side.