The new ships are extremely useful additions to the game and really give players an opportunity to try out new strategies.
Gunpowder- his bullets are ok, but his kegs pack quite a punch. They recharge fairly quick and have a decent hit size. He is pretty easy to pick up.
Treason- His special charges up to release up to 4 homing missiles (you can release sooner and release less though). I found him to be very tricky: while charging up you are very vulnerable and the rockets are not as heavy hitting as the Gunpowder's kegs, or the Bomber's explosive bullets. However if you want to focus on dodging and be guaranteed some damage, this is a nice character.
The Ghost- my personal favorite. He shoots a nice fan of bullets, but this special releases a ghost. The ghost can be planted anywhere on screen and shoots a fan in front and in back, and he remains stationary until you plant him somewhere else (or absorbs him). While the ghost is out, your character remains under your control but trades in his bullets for a short range beam (like a little sword).
Fortune- this is a character that randomly takes another characters abilities on spawn. He offers a nice roulette challenge for players.