I've had a good experience with Midwest Supplies on one of their kits. They ship very quickly, and their customer service is great (I bought a regulator add-on with the wrong threading [my mistake] and they took care of it promptly)
I got my tower and extra kegs from KegConnection. The price is good, their used kegs are in decent shape, and it's one of the few places where you can get a 100% stainless steel tower, but it took about a week to hear from them after I ordered.