Is this bag big enough for a weekend trip?

Yes for light packers. The main compartment takes a few days of clothing, and the exterior pockets hold shoes and toiletries. It is not sized for a week of travel.

Does it have a shoulder strap?

No. It carries by the top handles only, so it suits short carries from car to room rather than long walks through terminals.

Can it be used as an airline carry-on?

It is a soft duffel with no rigid frame, so it fits many overhead bins when not overpacked. Check your airline's cabin size limits, since a full bag bulges.

Is it good for the gym?

Yes. The roomy main compartment and separate pockets work well for a change of clothes, shoes and a towel, and it folds flat in a locker or closet.

How does it compare with a branded duffel?

It is lighter and more packable, but the fabric, zippers and hardware are more basic. A branded duffel gives better structure and durability for frequent travel.

Is it suitable for checked luggage?

It can be checked, but the soft sides and basic zippers offer little protection for fragile contents. A structured suitcase is safer for checked bags.