The magnets hold onto the tank of my bike very strongly. It is easily adapted to any steel motorcycle tank because of its design. I can fit several items in the bag and when i reach my destination it has a carrying strap so it can be carried on my shoulder. The bag comes with a waterproof cover to keep my stuff dry as well. Total capacity: I can fit two 24 oz. drinks (cans) and three 12 oz. drinks (cans). This is just to give someone an idea of the total volume of the bag.
I needed a bag with a GPS and phone holder that would fit my XR250's small tank. Most were way too big, this fits the bill nicely. It has 3 strong magnets that hold it securely to the tank. Inside space is surprisingly large for a "small" bag, and access is easy with the two zippers on the sides. Comes with a rain cover as well. A nice bag for the money and it is a great thing to have for my adventure rides.
Not much interior room here I use it for my phone chargers and personal hygiene products. The magnets are strong and I think the bag looks okay on my 3 gal sportster tank. I only plan on using it for long trips. It does fit my Samsung galaxy S II phone perfect (there wouldn't be room for a phone much bigger) and it puts it in a good spot so I can use the navigation app.
The GPS pocket is a great idea but a waterproof pocket/zipper would have been perfect. It will fit a 4" GPS but just barely. The GPS unit faces straight up so you have to lean over the bag and look directly down to see the display, sort of dangerous. If you want to run power or a headphone output, you'll have to make your own cuts inside the GPS pouch.
The cell phone pouch is fine and large enough for any cell phone from this decade.
Magnetic attachments are more than adequate to secure the bag.