I use my 105 2.8 underwater to shoot skittish critters I can't get close to with my 60mm macro. Excellent lens for studio close up when you want good bokeh. This is not just a macro lens but a good isolation lens, sharp primary subject, soft negative space (hence bokeh). Incredibly sharp, too. Here is a shot I could not have gotten without the 105 of a lizard fish.
I also have and use the big stopper. I shoot in RAW so I don't worry too much about color casts, but they do happen as kjorgan points out. Super easy to fix in post. You can easily overdo it with the big stopper. Creativity is your best friend. Frankly I am sick of shooting beach scenics with it. Here are a couple of shots with it. I love it, don't get me wrong. I just need to find new opportunities. Keep is covered at all times when not in use. Too expensive to just toss in your bag and then it gets chipped or scratched. Also you have to do all of your focusing before you put the unit on with the Lee adapter. You will need to obviously test for exposure and be cognizant of what motion you want to capture. This plays a huge role in how you compose an image. I have plenty of duds because I just did not capture the look in a way that spelled WOW. Here's a couple from my big stopper.