This depends on several things
• DVD is as standard (whatever program used) - interlaced SD-Video (far from progressiv HD)
• What material was used from start e.g. 1920x1080p50 - then resulting DVD will look like crap as this is down-scaled to SD-Video size then 50% is lost when trying to create an interlaced version of it (in iMovie'08 or 09 or 11), I use iMovie HD6 or FinalCut as they can deliver 100% interlaced SD-Video over to iDVD to do as good as can be.
Yours Bengt W