I think some here are generalizing a bit too much. My roommate and several friends of mine are committed house churchers, and whatever that is lacking in their theology and ecclesiology, they should be respected for honestly and sincerely striving to lead Christ-like lives. They take it seriously. I went to their house church meetings several times, and it is definitely possible to worship and discuss with them at a Mere Christianity-type level. This house church group (it's about 100-odd members, so they split into three groups on Sundays) is basically Baptist in theology. Of course, when they passed out the "Lord's Table" (grape juice and pita-type bread), I abstained.
Theirs is a misguided attempt to somehow replicate the "New Testament Church." So actual church buildings are out because the first Christians worshipped in homes---of course, that was because it was illegal, but no matter to them. In their historical reconstruction, they interpret the New Testament the way they need it to be, but they leave out the earliest noncanonical Christian writings which attest to things they would not ascribe to---the Real Presence, for example. I am tempted to ask them about the grape juice---in their supposedly painstaking historical reconstruction of "how it was in the beginning," why leave out the wine?
A big part of their church is a misinterpretation of the "priesthood of all believers." They claim that episcopal, sacerdotal or even pastoral authority is "not Biblical." So this kind of ecclesiology is the typical non-comformist Protestant kind that leads to a dizzying number of different groups, some less heretical than than others.
However, despite all this, there are elements of grace and truth that should be respected in these communities. And I know some wonderful Christian people, who truly lead lives of righteousness and take Jesus's teachings seriously, in these groups. Many of them are serious and sincere about their faith. Many hold firmly to traditional Christian moral teachings, the real miracles of Christ's birth and resurrection, and to a supernatural and transcendent God. I honestly feel closer to them than many "progressive" or "cafeteria" Catholics or Orthodox, who jettison these absolutely foundational Christian truths.
Of course, there are also the nutters, but we Catholics and Orthodox have many of our own nutters as well.
We should bring to them the fullness of the Christian faith by not rejecting outright everything in their current communities, for there is much good that exists there.