I have had my buzzbomb for about a year now, and it is a great cab. I use it vertical with my head on top (it is a small 20 watter). I have gone through quite a few different speakers in it, and right now I have Celestion G12K-100s. I am thinking about ordering another one to put my green machines in. Great cab, and lots of cool options. I have side recessed handles, side feet, and the casters built in to the bottom right corner for wheeling it around.
Great cabs, and great price, but if you are looking to add more low end more 12s won't do it too good. You might want to look at either 15s, or one of the powered subs from Rivera. Those are the best ways to add low end. You also might want to try eminence Man-o-War speakers. They are based on the T75, but they have the biggest low end I have heard in a guitar speaker!!!
One note about the wait times. Tim builds all the cabs himself, and the wait time issues came up while he was having some medical problems. From talking to him since he is doing much better after he went in for surgery. I have heard that the wait times are not a problem anymore. I would still say don't sell your cabs until you have the new one in hand. I sold my 412 to fund my buzzbomb, and I ordered mine when things started to get bad, and my wait time was about 8 months!!!! Tim was great about things, but I was left with just a single 112 cab for 8 months.