Interior space is a mixed bag compared with the other vehicles in the GLK-Class.   The BMW X3 has another inch of headroom in the front seats, though the Mercedes has more front legroom.  

The rear seats offer substantially more headroom than what's found in the BMW X3 or Infiniti EX, though legroom suffers in the Mercedes.   It seems like the GLK-Class gives up some rear legroom to gain front room, which seems like a good trade to us.   After all, back-seat riders typically don't make car payments, so they don't deserve as much legroom as you do.   But if you have tall, lanky teens in the family, you might want to look at that.  

Maximum cargo space is listed at 54.7 cubic feet, with the load compartment measuring 66 inches long.   That's slightly less than what's available in the BMW X3 or Infiniti EX.   The Lexus RX has vastly more cargo space due to its longer overall length.   The rear-seat backrests fold down in a 33/67 split or fold down completely to produce a flat load floor.   With the rear seats are up, the load compartment of the GLK350 has a capacity of 23.3 cubic feet when loaded to the top edge of the rear-seat backrests.  

An additional stowage compartment is hidden under the fold-up load floor.   This compartment is split into smaller partitions by a removable stowage tub.  

A safety partition net is available, handy for cargo and possibly dogs.   The load compartment can be hidden from sight thanks to the load compartment cover, while standard-fit load-securing rings make it easier to fasten down cargo.   There are bag hooks, coat hooks in the tailgate trim, a 12-volt power outlet and a lamp at the side of the load compartment.  

A power tailgate is available that can be opened and closed at the push of a button, either from the driver's seat or by remote control.