So, here are a few things to keep in mind when shopping around:
Brand - Many of the same companies that I mentioned in the cutting mat article also make a rotary blade. The three I've noticed most often are Olfa, Gingher, and Fiskars. Olfa is easily the most common and has the largest variety. If a quilt shop cuts their fabric with a rotary cutter, there's a 90% chance they'll be using an Olfa. Gingher only makes one type of rotary cutter, but it's built like a tank. Gingher is better known for their large variety of scissors (many of which are all metal construction and they're NICE). Make sure you're paying attention, because one is for righty's and the other for lefty's. My wife has a few Olfa's, but prefers her Ginger's. I believe part of it is what you get used to, because the instructors at Creative Fires swear by their Olfa's. In a not too distant third is Fiskars, not to say they aren't as good, I just haven't had the opportunity to use their rotary tool yet. Like Gingher, they're better known for their scissors.
Size - Olfa takes the cake when it comes to various blades, I've even seen a few decorative blades for pinking and scallops. Olfa has an 18mm and 28mm that are better suited for precise cuts and cutting curved quilt pieces, a 45mm which is the industry standard all purpose size, and a large 60mm that's good for cutting multiple layers or thicker materials (the larger blade should stay sharp longer, when cutting regular fabric, since its blade will contact the fabric less often). Fiskars offers multiple sizes as well but you may have difficulty finding the smaller replacement blades in stores. Gingher only offers the 45mm size, but that should be fine for most, plus they have some designer ones with fun prints on the handle. For a beginner, a 45mm blade is all you'll need to start, and I would recommend an Olfa because replacement blades are more affordable. You're bound to knick a few blades while you're learning to cut along a quilting ruler.
Handle - Like I said earlier, the type of handle you buy will really depend on what you get used to. Some manufacturer's try to create more ergonomic handles to make it more comfortable for longer cutting sessions or for those with arthritic hands. All brands and handles will have a safety cover that extends past the blade to help prevent accidents. Olfa has three main handles: their original which is held more vertically and pressed down, a deluxe rotary with a larger handle the reduces the angle you're cutting at a little, and their quick change rotary that cuts more horizontally like the Gingher model. Olfa's handles are a bright yellow hard plastic, while the Gingher is mostly metal like many of their scissors. I won't comment much on Fiskars since I haven't used them personally, but they have many choices comparable to Olfa.
Replacement Blades - Typically, you will buy the rotary handle of your choice and it will come with one blade. Once that blade becomes dull you can buy replacement blades, Olfa most commonly sells their straight blades in a five pack ranging from about $37 for the 45mm to $55 for the 60mm. Their original 45mm handle starts at $22 and goes up to about $50 for their 60mm ergonomic handle. Gingher is downright expensive at nearly $70 and their replacement 45mm blades are about $11 for just one! I'm checking these prices at Joann's online store, so remember to use the coupons they offer to save a considerable amount on your investments.
A few tips when learning to cut with a rotary blade... when you first start out you're bound to run your blade up onto your ruler or into the side of it. Do this enough and you will get a knick or dull spot in the blade that won't cut the fabric every time that part of the blade rolls over it. Watch out for pins and needles! These will definitely cause a knick in the blade, remove them before cutting a certain side or segment of your fabric. Also its best to use one long firm swipe or cut, don't run it back and forth like a saw, that'll just dull it more quickly. Press firmly, but not so hard that you cut your cutting mat in half, you'll get the feel pretty quickly. Don't fret, my wife says that by the time you finish your first project you'll be a pro.
Sometimes it's nice to save an older blade for cutting paper (not that you'll use it for that too often), many manufacturers say cutting paper can dull your blade more quickly. That also applies to many fabric shears as well, Gingher scissors shouldn't be used to cut paper, get a pair of Fiskar's crafting scissors for that. I've heard that cutting batting can dull your blade more quickly too, my wife prefers to cut it with her Gingher shears. Part of this is probably because she doesn't have a large enough mat to cut the width of it without moving the material. When it comes down to it ANYTHING you cut will dull your blades, its just about prolonging their life. A 60mm blade would probably be more suitable for batting if you don't mind the additional expense, but using your 45mm or scissors will be fine until you build up your notions stash. =)
One last thing worth mentioning. There are a few tools out there that are used to sharpen rotary blades to help extend their life. Dritz makes one that consists of two pieces that you fit the blade between and a coarse surface rubs over the blade as you turn it. My wife and I haven't had much luck with it, but our blade was knicked, so if it were just dull we may have faired better. Also, my wife saw on Pinterest that you can cut aluminum foil to help sharpen your blade... haven't tried that either, so try at your own risk.
Hope this article helps everyone, feel free to add your input in the comments too so we can all learn!