A little history
Almost as revolutionary as the invention of the ascender for moving yourself up a rope was the realization you could turn an ascender upside down, run a rope through a pulley and pull down on it, thus efficiently hauling gear on big walls. Then, a few decades ago, big wall hauling devices were invented that mostly relegated the old pulley and ascender method to the sidelines.
The main advantage of the pulley and ascender method is for loads over 200 pounds. For loads that big, you want a pulley with a sheave diameter of more than 2 inches. At 2.4 inches, the CMI pulley is much bigger the Pro Traxion or Wall Haulers 1.5 inches. How much is 200 pounds? Well, a party of three that is starting a four-night El Cap ascent could easily have that much. They would really appreciate the big pulley down low. Up high, as the load got lighter, the advantages of the other hauling devices would become more apparent. For truly massive loads, you might want a 3 to 4-inch pulley. But at that point you have to ask yourself if you really want to bring that much stuff!
The other nice like thing about the pulley and ascender method is price. Since you will probably already have a pair of ascenders, all you need is a pulley. You can pick up the CMI RP 101 for $28 or you can usually scrounge up a pulley from a friend (they are a lot easier to scrounge than wall hauling devices).
The main downside is the time and hassle of setting up the haul. You need to get the ascender in just the right position, get a counterweight, and get the pulley running perfectly aligned with the ascender. It takes more time than using a hauling device. An extra three minutes might not sound like much, but if you are trying to do a wall in one or two nights, every time you have to spend extra time on something like setting up the haul adds up over the course of many pitches.
Once the bag gets to the anchor, you won't be able to clip the bag up as high on the anchor as you can with a wall hauling device. Another big downside is you can't space haul unless you bring a third ascender. And finally, with this method, you always have to lead with two ascenders and a pulley. That is much heavier than leading with one ascender and a hauling device. If you are doing the Nose, the difference between two ascenders and a pulley and one ascender and a MiniTraxion is more than a pound.
The best application is on big walls where you are hauling a ton of gear… which we don't recommend doing!
If you already own ascenders, this is the cheapest hauling option because you just have to borrow or buy the pulley.
CMI makes a lot of difference versions of this. Most of the variations are the same size but are more expensive because they use a fancier sheave. We just stick with the cheap version because it works great at half the price. But if you want to see other versions, just search CMI Pulley and look at the models that are 2 and 3/8 inch.