My opinion: Essentially a Patrol Ship with a Rocket Launcher, the Gunboats price just doesn't make it worth buying. See Manta
Computer controled: These ships are twice as annoying as Patrol Ships. They're easy to kill but will always get off their missles. Avoid close contact with Gunboats, they'll always launch their rockets at you. If you're able to render one of these disabled (which is hard — you'll probably end up blowing it up) they're very easy to capture and make a good addition to your fleet.