I have also heard some negative things about Alpha Course. Last year I went to an ecumenical gathering at Stuttgart. Nicky Gumbel was one of the speakers. Everyone else was giving speeches about peace and unity, the usual stuff, but Nicky Gumbel stuck out like a sore thumb. His speech was primarily about how great Alpha Course is, it sounded like a sales pitch and did not fit into the program. When I saw his name on the list upon arriving I was puzzled, his speech confirmed my doubts.
Here is one article by Chris Hand:
http://www.intotruth.org/misc/alpha.html .
And another by Tricia Tillin:
http://www.intotruth.org/ms/ms2962.html .
The course has a Pentecostal/Charismatic slant to it.
There is even a Catholic office for Alpha Course.
If you decide to take the course anyway (which I would not recommend) I suggest you read the articles listed above and keep them in the back of your mind at the same time. Keep your mind clear to observe, do not just feel what is going on around you.