I purchased this one at cyclegear.com; it was fairly reasonably priced and the magnets hold it in place nicely. I use the clear window for maps, it actually works well. It also expands quite a bit.
$63.17 w/ free shipping: [ame="http://www.amazon.com/Nelson-Rigg-CL-1035-MG-Black-Magnetic-Touring/dp/B00AC6Y9JK"]Amazon.com: Nelson-Rigg (CL-1035-MG) Black Magnetic Mount Touring Tank Bag: Automotive[/ame]
Just picked up a matching Cortech tank ($72) and tail bag ($110) this past weekend. The most important thing is the 10lbs weight limit that these bags recommend, so you'll never fill +30L unless it's with balloons. Also recommend buying a matching tank and tail bag at the same time, for weight distribution. Soft saddlebags are pointless, but a soft tank and tail bag gets the job done quite nicely.
Map windows are awesome. Had my iPhone with Google maps running and it was great. Recommend a bluetooth headset so you can synch it to your phone to hear the turn-by-turn instructions.
Also make sure the bag itself is waterproof, including the map window. The Cortech one comes with a rain cover, which unfortunately makes the map window useless if you want to keep the bag dry.