Yeah I agree. We picked a small one up for my younger brothers years ago. It was the best thing ever for a week. Its been in the shed for the last 12 years. Don't even know if it still works. Would be a nice project to make though so long as you can easily turn it into a coffee table afterward.
Building your own air hockey table seems like a lot of work, are they that expensive that you'll have to consider it? I like air hockey but couldn't see one in our home because they take up too much space and the smaller ones are not as fun.