I haven't delt with them. I have read that some of their stuff can have fitment issues. Namely, I was after their light bar since they offered one without the brake light for less, instead of more for the delete. Saw some pictures of the light bars from them installed, and they fit like ****. Gaps everywhere. In that same thread, everyone kept pointing to Classic Design Concepts.
Over all though, i can't say whether they are good or bad. I have only heard to stay away from their light bars so far.
There will be some fitting to be done with all body kits regardless, it's just a matter of which kits are the closest to fitting correctly. I've heard good things about classic design concepts, but you can never outdo cervinis....
Use them only if you want to pay a couple of grand trying to get a bumper just to fit right. They are the ones that sell the fiberglass kits you can see through on eBay.
Cervini's is on top. They make the best stuff. MrBodyKit is at the bottom. ABC is in the middle. They make good stuff, the make bad stuff. If the kit you want is made by Cervini's, go through them. It may cost more, but you'll save on the prep work.