I've had the Ultra 30 tank bag for a little over a year now. In general I'm pretty happy with it although it does have a few shortcomings.
First off, for the price ($160 list, which is way more than I paid), the bag should come with the rain cover as standard equipment. Not as a $20 option.
Second, while the bag is solidly made, they didn't seem to take into account the "weatherability" of the bag. By this I mean the ability to keep out rain during light sprinkles. I don't expect a bag to remain watertight during downpours but I should be able to ride a few miles during a sprinkle, on a faired bike no less, without rain entering the bag and/or map pocket area. The problem is that Marsee uses these very stylish and easy to work, large toothed zippers. The large gap of the teeth gives water an easy route to the inner bag. To exacerbate the problem Marsee neglected to sew in any kind of zipper flap to cover the zipper itself. The addition of the flap would remove a large part of my hesitation in recommending this bag.
While it hasn't proved a problem yet, the map pocket is a permanent part of the bag and this could prove problematic. I worried about this before I bought it since I've seen map pockets ruined by UV rays. But Marsee assured me that they use a plastic with UV protection and I should see a long life from it. A permanent map pocket also makes it impossible to quickly detach your maps for the inevitable route discussion that takes place at all eating stops.
On the positive side, the mounting method Marsee uses on the non-magnetic bags can't be beat. The bag uses a base which zips on and off the bag to attach to the bike. All the normal stress points that are usually directed to the bag itself are now transferred solely to the base. Two nylon straps run the full length of the base to create the four attachment points. (Check out the picture at the Marsee website.) There is no way this bag is going anywhere and there is no chance that you'll pull bag seams by over fastening the bag to the bike.
The entire Marsee bag line also has a few features I'd like to see other mfgs adopt.
Net bottom line is that I have a hard time recommending this bag - even taking into account the fantastic mounting method. It's nice, but the bang for the buck ratio is way out of line. I think Chase-Harper, or even RKA, gives you just as good a bag for less money.