Man, if you're starting out and you know you'll need several components for a bag... just get this. It will save you so much time picking everything out that even if you balk at the price (amazing value, by the way) I bet that if you did the math using your hourly wage and the time spent picking everything out and making sure it fits... you'd save a lot of money and a lot of irritation if you just got this. Just get this.
Otherwise, this tool should serve the average commuter well. Specifically, we think you’ll find the size 4, 5, and 6 hex keys, extremely common sizes in bicycles, very helpful. They’ll adjust seat-post heights or let you remove the saddle entirely, or tighten a loose stem that’s always rattling apart. The Phillips head will tighten loose bolts on shoe cleats or the rear derailleur. The most common use for the torx would be adjusting disc brakes if you have ’em.
It includes nine tools: seven hex bolts, one torx bolt (the star-shaped one), and a Phillips screwdriver (also the star-shaped one). Like Peter says in our guide, if you have a newer mountain bike or road bike it pays to take a quick look at what types of tools you need, as torx bolts are becoming more common. And a quick glance at the bottom of your shoes or derailleur bolts will confirm if a Phillips head is the right choice.
2) On my second day, the pannier bag fell off and got lost after I stopped for lunch, which required locking up the bike somewhere, so I took the bag into restaurant and re-attached it using the two velcro straps on the rear rack. I've used many panniers before, thought I did it correctly. But apparently I did not. So I paid $55 (cost) to replace everything in bag and bag. In hindsight, I wished I'd put a bungee around bag straps to really secure it to bike. PROBABLY MY FAULT.

These guys are awesome.  After work, I got to my bike to discover my back tire was completely flat.  It had a slow leak, so I was able to pump it up enough to make it to their shop.  I got there 10 minutes before closing and they were able to fit me in.  They replaced my tube and picked all of the glass shards out of my tire.  Awesome customer service and they saved me from having to do this myself in the dark and the cold.
Packed with basic tools, this compact tin rescues puncture-interrupted bike trips. Pedaling away to a picnic spot with a pal, you hear a disconcerting "pop!". Your two-wheeler is now down one wheel, and unless you've got mad unicycle skills, you need tools. Good thing you brought this handsome, compact tin. Just break out the metal tire levers and patch kit, and you're rolling along again in no time. The six-sided, dog bone-style hex wrench adjusts the most common bike screws and bolts and screws. Made in China.
Brakes come in a variety of designs depending on the age and style of your bike. Rim brakes offer more modulation but have less stopping power, so are more suited to riding on the roads. Disc brakes can provide huge amounts of stopping power so are best suited to off road riding, although ‘V’ rim brakes are also well suited to mountain biking. All brakes use a friction system to stop the movement of the wheel, so regular checks of the contact pads should form part of your regular cycle maintenance.