I really do love this small pack. On day trips it holds what I need, a shell, gorp, cell phone (I know!), 1st aid, keys, large bush knife...It seems every item I want to carry, I can, since there always seems to be an extra place. Paired up with almost a full gallon of water on your back, this is a powerful tripper in a small space.
I bought my MULE back in 1998 and have used it for many mountain bike outings since then. I find it to be the perfect size for mountain biking, as it easily holds the full water bladder, together with two extra tubes, a compact pump, an assortment of tools, a cell phone, and my wallet, with room to spare. It fits so well that you often forget you're wearing it.
Complaints: the valve for the bladder tube is starting to leak, so my shirt gets really wet when I ride. I've corrected this somewhat by routing the tube up higher.
The MULE is the perfect size for day hikes here in Colorado. There is enough room for a light rain jacket, extra fleece and food for the day. There are tons of pockets and compartments to store items. 100 oz water bladder seems to be just enought for the day.
I also use this for 12 hour or less adventure races and have found it to be more than adequate.
I agree that this pack is a nice peace of equipment for short daytrips (not necessarily mountainbiking only): comfortable to wear, light, can accommodate all essentials. The only con that I can think of is a possible LEAK of the water bladder. I have used MULE quite intensively for a couple of years, with and without the bladder...Then decided to use the same bladder in mountaineering trips with a bigger backback designed for water reservoir. Indeed, it is much more comfortable than messing around with your water bottles all the time...Then once during a multi-day trip the bladder started to leak from an opening screw without no obvious reason. I don't know exactly what was the cause (maybe overtightning of the screw?), but the result was not pleasant at all. It helped a bit to have a half-fill (anyway, in a backback everything gets pressed together, so does the water bladder, and even if half full, water still reaches the opening). After that experience had to replace the reservoir with a new one. MULE itself (even with bladder) gets 5 stars indeed!
A very handy little pack for summertime day hiking when water is essential but little gear or extra clothing is needed. I have used this pack for hiking and travel in the Franklin Mountains and Hueco Tanks of west Texas, White Sands NM, and Carlsbad Caverns NP. Lots of pockets and organization, although the main pocket is smaller and difficult to use when the bladder is full. I can easily fit a light shell, lunch, sunscreen, digital camera, and various odds and ends.
Camelbak sets the standard for hydration systems. The MULE is perfect if you want to take 100oz. of water and only need minimum storage capacity for gear. The waist belt is nice and keeps it comfortable.
I have used the MULE for years, particularly for trail running when I go out for more than an hour. 100 ounces is great to have when you don't know how long you'll be out. The pack has enough room for sunscreen, sunglasses, maps, compass, gels, keys, wallet and light windbreaker. I always use it when running ultramarathons so I can hydrate between water stations.
For backpacking and day hiking, I take the bladder out of its pack and put in my backpack or daypack. I find the MULE pack too small by itself to accomodate what I take on day-long hikes.
The only problem I've ever had with it was when once the hose at the bottom of the bladder disconnected. I lost all my water on a very hot day. There was no defect; the hose had just worked loose. The solution? I now check that bottom connection every time I use the bladder to make sure the hose is on securely. Other than that one time, I've not had problems again with it working loose.