Ghost
Caution: floaty ride
Personally I find this hovering purple bath-tub way too floaty and unstable to enjoy much. It's slow to respond to control, and easy to accidentally overturn. Used carefully though, it's pretty effective, and an Elite in his own bath-tub should be no match for you. For example you can circle him whilst maintaining your aim and piling in the plasma; he's slow to rotate and shoot back.
Anti-Ghost tactics
Most weapons will soon start a Ghost smoking nicely. However, going for the Elite may neutralize the threat quicker, and leaves the Ghost useable if you're interested. When cornered on foot by a Ghost, you could try getting behind an obstacle and then popping your head out to tag the Ghost with a plasma grenade. Alternatively, a fast-exploding frag grenade might cause the Elite to be ejected if you're lucky.
Lethal Marines
Because Marines have a nasty habit of clambering into Ghosts and then running you over, which can be a touch annoying, you may sometimes like to grenade Ghosts beforehand so they're overturned. Neither Marines nor Elites can right an overturned Ghost to use it.
Stealing a ride
Something you may not have realized is that you can make a Marine dismount from a Ghost by getting close and hitting the 'X' button when alerted on the screen. You can then take over if you want. But it's a bit risky getting close of course. Try it with the first Ghosts in level 5 if you want to check; that's where I eventually noticed the possibility (I happened to be next to the Ghost when a Marine jumped in).
Raising the nose
Holding the 'A' button causes the nose to raise, which can help you get over higher obstacles. It's also handy for stopping the nose untidily scraping against sudden rises, for example when navigating the up-and-down tunnel flooring in the underground bridge section of level 5.