I only have one MIK bag:
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Buy BiKase MIK Big Poppa Extra Large Expandable Rack Bag with Panniers, Ideal for E-bikes & Bicycles - Waterproof Rear Bike Trunk with Ice Compartment, Phone & Bottle Pockets, Reflective for Safety.: Panniers & Rack Trunks - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
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I bought it for it's size. With it's fold out panniers, it holds quite a bit more than the large Topeak bag. I don't see much difference in quality except the zippers aren't quite as good as the Topeak.
Yeah, the MIK lock is nothing special. It's recessed under the rack and requires a long key to release. Could be opened with a long thin screwdriver if you know how. Won't stop a determined thief but gives the casual ones a reason to move on to easier prey.