I tried Levelator out about a year ago when this forum went through the same discussion and it was recommended.
I was not entirely impressed as they are applying dynamics compression so my caveat continues that unless you are finishing the audio in FCP (something I never recommend) then do no do processing like normalising or using Levelator if you are then exporting an OMF for proper audio post production as you affect the clips in the OMF unlike applying plugin filters which are ignored by an OMF export.
The quality of dynamics control that I am used to with professional mixing systems like Fairlight are better than Levelator which does an excellent job but is not in the same league of a well driven dynamics module.