For seriously underexposed shots 'Levels' is very good, and I'd say much better than the colour corrector. I can't say how/why, but it's offseting the video signals in such a way as to keep darker areas dark, and colours more realistic.
Try it on a normally exposed shot, blow it out using both methods. The levels method is much more controllable as it's only one fader, and for me it's the tool of choice.
Levels as a whole are an amazing plugin for fixing very bad white balances, like daylight to tungsten etc. Of course this is with the Nattress filter I bought for FCP6. I'm still waiting to use FCPX on a paid gig.
The exposure control fader in FCPX is poor, nothing like it's counterpart in photoshop which seems to act true to it's name.
YMMV
Cheers, Duncan.