Terrible. The Hammerhead is a giant step back from the Mako and the short missions were utterly pointless. I don't care if it was free with the game I got, it was terrible and I often feel like it is a waste of time and drive space.
Firewalker is a DLC used to show off the Hammerhead, a vehicle which is a vast improvement over the Mako in terms of manoeuvrability and firepower. It is also nice to have more open area to explore seeing as most ME2 missions are linear.
Felt like an amendment to fan's disappointment to a lack of vehicle sections in Mass 2. The Hammerhead isn't the Mako, and can fly (hover?) in a straight line. Essentially a re-hash of the explorable planets from Mass 1.