The Unlimited Gathering Tools aren't always available in the store, and I am borderline idiotic when it comes to buying replacement tools, so those were all more than worth it for me on my most-played character. I should really consider grabbing one or two for alts as well before they're gone...What's worth buying in the gem store? The Salvage-O-Matic and unlimited gathering tools are intriguing. I'm really digging the game and plan on playing for awhile so I'll probably drop some cash into it soon.
The salvage-o-matic is mad convenient now that the blues and greens of the world are meant to be salvage.
Depending on your armor class some of the armors are actually pretty sweet.
I find merchant, TP, and bank expresses to come in handy, but you can pick up a fair few of them without spending any gems.
Inventory slots, bank slots, and character slots.