I didn't know these guys were still around! They're mentioned in an old Mercedes tuning book from the 80s/early 90s. I remember (from the book) they used to do E500/500E conversions for normal W124s, as well as full interior retrims. The book showed a 190E and W124 interior. Both light grey with grougeous hand stitching I'll list the name/ISBN of the book, if anyone's interested?
It is a replacement skin for the oem bumper frame so it's not going to be that bad. I see quite a lot of reiger kitted cars knocking around (boy racers) and although the cars themselves look ugly the bodykits look great from a quality point of view. If it's a reputable bodyshop then it will be fine. I believe that Brabus, Carlsson and lots of other tuning companies use fibreglass skins for styling packages.
As for the fitment, like you said, since rieger are quite common now maybe they do fit well. If rieger produced a more subtle range of kits (and for a similar price) I am sure they would sell much more