If you ride one of the newer-model bikes with little to no rear storage space then this bag will bascially get you back what the underseat exhaust stole from you - and not much more.
This bag is compact - there isn't even enough room in its main compartment to hold a spare visor. There is room for a small bottle of water, your wallet, etc. But if you are a commuter you may have to decide whether to bring your lunch or your jacket's rain liner because there isn't likely to be room for both.
A couple of exterior pockets leave room for keys, an MP3 player, or phone and this is helpful since they probably add about 20% to the overall capacity of this unit.
Magnets are super strong and the bag clings tenaciously to your tank. The "manta" shaped side magnet wings are pitched towards the rear of the tank so there are no worries about attachment on bikes with a half-plastic fuel tank.
The bag comes with a convenient strap and also a rain cover that I have found to work quite well even in a pounding rainstorm. Overall build quality appears to be very good.
There are some distinct advantages to a compact tankbag like the Manta; it won't get in your way when ripping through the twisties and the small profile looks really good on modern bikes.
But if you live on your bike day in and day out, commuting during the week and ripping upo the hills on the weekends then get the Manta for the weekend rides and something a little bigger (and more practical) for the daily grind.