I've mostly been using my rotary cutter to cut out my fabric pieces, because it makes such nice, straight cuts and my scissor cuts come out all ragged. Plus, it's much faster! However, there is also much higher potential for major screw-ups and/or major injury if you don't use it properly, which is why I'm glad I found this tutorial from Purl Bee before I cut too much more fabric!
Naturally, it turns out I've been developing terrible rotary cutting habits, like cutting towards myself when I don't want to deal with trying to get around to the back of my table. But this tutorial explains all the proper techniques - like how to hold the cutter so you don't get carpal tunnel, and how not to injure yourself - as well as the right rulers to use with it (hint: not my slippery metal graphic design ruler). I've held off until now on buying all those crazy sewing rulers because I wasn't sure which ones I really needed, so this is very helpful.