dinan has been in the game for a long time, but i say it's a coin flip. some people say dinan will fix ur car if something goes wrong. others say dinan will come up with excuses so they won't fix ur car. there are also people saying that you can only get dinan products. any other aftermarket items non-dinan will void both warranties. you'll also have to look at price factor/performance.
go to the e90post forums here
http://www.e90post.com/forums/forumdisplay.php?f=58
there are a lot of discussions regarding dinan that i've gone over a bit here.