Longer trips this Summer had me trying a couple of different seat bags. I settled on the Revelate Designs Pika which is the smallest of the 3 models that Revelate offer. It's super easy to attach to the seat and the buckles have locks to stop them from loosening off after heaps of km's of vibration and abuse.
The Pika can carry a lot of kit and I found it best for lighter bulky items but just as practical as the larder. The Bungy attachments were handy even if you do have to provide your own cord and clasp.
On techy trips I prefer to pack lighter and be able to use the dropper but on extended trips or on gravel bashing or 4WD tracks a seat bag is an ideal way of carrying more gear and the Revelate Designs Pika is a superb bag that once installed you'll seldom feel it behind you.