Valin,

Some of my clients have them and some don't.  I do not have one on my Legacy.  What I have noticed is if you like to run the airplane with the oil near full the air/oil separator helps.  Some of the reasons guys want to run very full oil is to keep the engine oil temp lower (masking a problem IMO).  What I have found, if you run the oil to its "happy level" and leave it there you will see very little oil overflow.  For example, my engine likes 6.5 qts and will stay between 6~6.5 for about 25~30 hours.  When the oil goes below the 6.0 mark I add a half of a quart.  Now some of my clients will run to 7+ with an 8qt sump and it will pump oil out right away.  One of my guys has a 12 qt sump and his airplane likes to stay around 9.5.  He had a small air/oil separator early on.  When he was trying to run at 11-12 qts he ended pumping oil out with the separator at about 1 qt every 5 hours.  He then went to a large separator and that reduced the rate of oil going overboard but over 25 hours it would still pump oil until it got to it's "happy level."   We went back to the small separator and got him to run max level at 9.5 and he uses very little oil now. 

One issue that I have noticed in the 550N world, the dip sticks are often wrong.  It is important to calibrate the oil dip stick.  I calibrate the dip stick to the amount in the sump not the complete engine.  When you do your oil change and drain the oil, check the stick as you fill the sump.  This way you know how much is in the sump.  Every engine is different with respect to accessories.  For example large oil filter, different prop, different governor, external oil filter, ect.  So having a dip stick that is calibrated to include these items is not that important.  This means once you run the engine after the oil change you will have to add oil to bring the sump back up to it's proper level.

My opinion is that an oil separator just slows the rate down but doesn't eliminate oil overflow.  If you like a really clean belly, I find running your oil at the "happy level" and a small separator will keep the belly completely clean.