ADC Student Member, which costs $99 per year (although you need do it for only one year), you get a One-Time Hardware Discount on buying a Mac. This is one whopper of a discount, make no mistake. It's not very useful if you're thinking of buying a MacBook or a Mac mini (because it's only another $50 or so less than the standard educational discount), but, if you're planning to purchase an expensive Apple computer like a MacBook Pro and also happen to be a student above 18 years of age, you're in fine luck. However, One of the conditions of the One-Time Hardware Discount is that, well, it's one-time... per life.
Let me give you an example of the discounts. The usual MacBook Pro which sells for $1999 normally, is sold for $1799 at the regular Apple Education stores and for $1599 at the ADC Hardware Purchase Store. The $2499 model is similarly sold for $1999 and the $2799 for $2239. So the discount you get more than makes up for the $99 you spent on the membership.
But, that's not all. Another benefit of this membership is that "Upon purchase and activation, you’ll receive Mac OS X and Xcode Tools". Now, I don't know if that means, if I buy it today, I will receive Tiger now and any new versions of Mac OS X (*cough* Leopard *cough*) when they come out during the course of my membership, or whether I'll just get Tiger. Since the membership is $99 and Leopard is probably going to be $129, if their terms and conditions are still the same when Leopard comes out, I'll go ahead and buy this instead of Leopard and get it for $30 less, in addition to a voucher for an extremely sweet One-Time Hardware Discount.