Destro can be fun especially if your team plays with/around having you, coordinating chaos bolts with swaps etc. you have limited snares and spreading immolate is "ok" damage, conflagration can help secure kills a lot. You suffer from very low mobility and no escape mechanics if you don't play goblin. I really enjoy demo, your dots don't do a lot of damage but you can spread them like a boomy and leap around and get fears on people. Passing out health stones is pretty great, and soulstoning a healer or frontline in pugs. You can combat bug like a hunter but it takes more work, and your pets die faster.
Arcane Mage has a lot higher mobility, consistently high damage, counterspell, ice block... You can triple DR sheep a kill target to keep them in place etc. Frost can be good with smart pet usage. Can also combat bug like warlock, and snares are always nice. Frost is great in arena.
Overall Mage brings more to the table but if you find warlock fun definitely give it a shot. It's hard to see what a warlock brings over a boomkin sometimes though.