Scorpion
Clumsy but powerful beast
Available only in level 5, the Scorpion certainly looks tough, but then you notice that the incoming fire is knocking seven bells out of your shield, despite the hefty armour supposedly protecting you. Bit of a design oversight I'd say! It's also slow, making you particularly vulnerable to plasma balls from Jackals, or plasma grenades from nearby Grunts.
Another shortcoming is that you can't independently rotate the turret when driving, because motion is always towards where the gun is pointing. This makes strafing impossible, forcing you into potentially more risky head-on motion. I've always found the gun targeting rather hit-and-miss, which can be frustrating, and as for the machine gun, that thing scatters bullets so widely it's almost too painful for me to use.
In view of all that, I have to say that the Scorpion is rather a clumsy beast. However, if you operate carefully it can certainly be good fun, and the hugely powerful blast from the gun is of course quite satisfying, taking out Ghosts or Banshees in one big bang.
Marine outriders
Up to four Marines can ride on the sides, and a sniper (preferably at the front) makes a highly valuable addition. Normally, outriders will dismount a few seconds after you bail. However, that behaviour only actually operates once the skirmish in the crashed Pelican area has been triggered. Prior to that time, any outriders will simply stay aboard. And bear in mind, it is actually possible to have the Scorpion prior to that trigger, with suitably nonstandard gameplay. For example you can descend from the first bridge and bring the Scorpion to the first snowfield for subsequent use when you trigger the battle there.