The gun is better than the last time, I like the new varieties of weapons, such as fraggo and little friend as well as new punchy sounds. Also, it looks like there are now weapon rarities, I'd suggest drawing an outline around the weapon to show how rare it is, cause you can only tell it by standing on it and looking a the stats(stats display is a nice addition, too).
I never had a reason to buy any weapons at the store, cause they were pretty similar to those that get dropped during the levels, so I only bought ammo.
The game feels like it became a lot more difficult, and I usually die in one of the cave levels when the enemies box me into a corner, I haven't even made it to the boss yet.
The upgrades still feel as incremental as the last time, their effect is barely noticeable.