I'm always seeing different kinds of packs for mountain biking and eventually I'll need to buy one. What kinds of packs are you using? Lumbar or backpack? How many liters would you recommend?
There are pros and cons to both lumbar and backpack packs. You are looking at hydration packs right?
Backpacks are typically larger and can hold more water and gear (although not always). Backpack style packs usually have a bladder that can hold 1.5-3.0 liters of water. Then you can get one with enough room for a jacket, tools, snacks etc. So a pro is that it can haul a lot of water and gear (Camelback HAWG reference) which is usefull if you are going on longer rides or multiday trips. At the same time, all that stuff weighs more which can mess with your center of gravity, goofing up your riding postion. Also, it covers a large area on your back which makes you hot and sweaty. A last positive is that I also believe that a hydration bladder in a full pack offers some protection in a crash.
Lumbar hydration packs can usually hold between one small water bottle and 1.5 liters of water in a bladder. They sometimes have room for some basics like a multitool and small snacks but usually not a light jacket. This makes them lighter so they mess with your center of gravity less and smaller, creating less heat and sweat on your back. Negatives are not being able to carry as much gear as you may need or as much water as you might want. Typically you can mount at least one bottle onto your frame and can store more gear in a frame or seatpost bag, eliminating the need for a pack at all on the shorter rides that a lumbar pack might be usefull. For lumbar packs with a hydration bladder, I haven't had or seen one that secures the hose in a satisfactory way in which I don't think that it might fall loose (this can and does happen all the time) and into the front or rear wheel and ruin your day (or month).
I would lean towards a hydration pack with a 3.0 liter bladder for longer rides and bottle cages frame/saddle bags on shorter rides if you want to stay cool. I wear my pack on shorter rides too, just with less water for the protection.
I own and use both depending on the trip. Lumbar is the way to go for ease of use, comfort and a dryer back. I recently purchased the new Osprey lumbar pack (seral), Its got a 1.5 liter bladder and room for tools, snacks and a jacket (a must for summer alpine rides in Colorado). I also use a water bottle in the frame and can easily do rides of 40 miles with water to spare. I also owned the Evol lumbar pack but returned it because of poor bladder design and it was just a bit to small for my needs.