I had a Vstrom 650 with plastic Givi cases side and top. I loved those cases. Very light weight, water-proof, locking, and durable--very easy to work with. A snap to take off the bike and take into motel room--look like suitcases basically. You can pack your cases in your comfy temperature controlled motel room then at the last minute take them out and snap them onto the bike.
The Great Basin bags are a different deal completely. They are for when you will be riding stuff rough enough that it matters to keep that weight in tight and low. They are for when you are riding stuff rough enough that laying the bike down a few times is inevitable.
I love my Giant Loop Great Basin bag, but I wish I had my Givi cases for more relaxed travels or around town.
I've never owned the aluminum cases, and I know there are 50,000 riders ready to defend them to the death, but from my naive viewpoint, they are heavy, expensive, only waterproof until you bend them, and square corners could do some wicked damage in an accident. The argument for aluminum cases over plastic (like Givi) is their durability. However, I've done the math, and I could smash 3 plastic Givi cases to one common aluminum case. Also note that I did in fact break a Givi case off on a stump along the trail. It worked perfectly as designed--the latch mechanism broke (as designed) which allowed the case to come off---virtually undamaged. I just hooked it back onto the mounting tabs and used a bungee cord to hold it onto the bike for the rest of the ride. Givi sells replacement latches for $15, so I was good as new after a 5 minute repair.
The moral of the story is that for me personally, I want plastic, rounded cases for casual riding. When I get into stuff that demands the strength of aluminum cases, I'd rather have the Great Basin bag.
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