The Old Order Amish still worship mainly in church member's homes. This is a custom that goes back centuries to when the Anabaptists were persecuted for their beliefs and driven "underground" in Europe. Rather than deal with government harassment, the Amish secretly began worshiping in church member's homes where the government or anti-Anabaptist forces could never interfere again. Fast-forward centuries to the United States in 2013 and you don't have persecution (at least not in the same direct way...some would argue that government laws curtailing certain practices or imposing onerous requirements does amount to a form of persecution, but that is a post for another day), but the Amish still generally worship in their homes.
Of course, today there's no attempt to worship in hiding. One can drive the rural roads of Amish country on a Sunday morning and easily pick out where church services are being held by the sea of buggies. The custom of home-worship has felt fast, but there are some signs that is beginning to change. Do all Old Order groups worship in their homes? Old Order Mennonites, for the most part, worship in church buildings known as meetinghouses. As do the Old German Baptist Brethren. The Amish still worship in their homes with a few notable exceptions.
The Amish settlement in Unity, Maine has a church building. When I visited in 2013 it was still under construction. Look at the nice view of the surrounding Maine countryside from the church balcony! Having a church building makes it easier for Seekers or outsiders to come and check out the faith. Most Old Order Amish churches eschew outreach and evangelism so not having a meetinghouse is one way to keep that in check. This group in Maine is a bit more robust in their outreach. "We try to serve as a witness for others," is how one Old Order congregant put it. I am aware of a few other places where horse-and-buggy Amish maintain a formal church building, but the vast majority still worship in their homes. Below is a German Baptist Brethren meeting house on Diamond Mill Road in Montgomery County, Ohio
One trend that is catching on among many groups of Amish is a "community building." These are used for community potlucks, singings, youth activities, auctions and some are even used for funeral viewings and wedding gatherings. Some more conservative Amish fear that these buildings could be the beginning of a slow trend towards moving worship away from homes and into meetinghouses. Other Amish would welcome that trend, because preparing one's home for a large church gathering is a large undertaking that usually falls to a family once or twice a year.