Yep, that's a nice service. Just be sure to match your components to the case in terms of fit. pcpartpicker.com is a great resource to check compatibility.You can always by a case elsewhere and send it in to PCS - they offer that option.
Then simply buy the case else where, a simple google shows a few places have them.I am waiting on the Fractal Design Define 7 which was in stock a few days ago. It is a shame there is no option to select it as a pre-order and be placed on waiting list. I can't even place an order now. Very disappointing!
Corsair cases are very popular.It seems it is changing, there is one Be Quiet case showing now.
There are 7 Corsair cases available ranging from £44 to £175. Are they really unpopular or they just happened to get a large delivery of them or had a lot of existing stock? I imagine we will probably never know if it's the latter two, searching throw the forums will likely show if they are popular or not.