I agree that CA, at least the "gap-filling" type, works very well for filling gaps (as long as they are not too big). A couple of quick points on using CA as a gap filler --
I have found that many times it works better to use gap-filling CA along with a CA accelerator (usually in a mini-squirt bottle). Once in a while when gap filling CA is allowed to dry normally over a longer period of time (i.e. overnight), the joint or seam that I was working on ends up with a rough texture with many small voids and gaps in the joint . I haven't figured out why this sometimes occurs (other times it works fine and is nice and smooth). Please note that the accelerators can often do nasty things to your paint job, however, if the part has already been painted.
I also have found (and heard on the forum) that it is much easier to do the filing or sanding of the CA filling the gap very soon after the glue sets, but before it fully cures and hardens (I usually begin sanding and filing after only a couple of minutes if I use CA accelerator or after 10-15 minutes if I am using the "10-15 second" gap filling glue and letting it dry without accelerator). The CA gets pretty hard once it fully cures and takes a lot more time to sand or file sufficiently, even if you do not use excessive glue.
Just my two cents.