i use managed descents only for stars with multiple constraints and it is much easier for the fmgc to get it right than for me to manually figure out the profile. in such a situation having the extra n1 is OK.
however, landing into a field with no traffic and a straight in ils apporach or something similar, i start my descent on vs, not more than 3 miles beyond the tod arrow. start with a vs of 1500 progressively increasing it in increments of 500 to a vs of about 3000-3500. flying at fl390 at .79 indicated speed is around 245 and our descent speeds lie around 305-315 knots for the company cost index. so taking open descent at tod has the probability of the vs going to 6000 which is really really uncomfortable. so descent on vs till mach changes to speed and then take open descent. at this point my vdev is around +1500 and is very comfortable to to be descending in open descent and configuring and be stabilised by 1000 feet. there are a few occasions where you will have a level segment but taking it on vs helps avoid it. 1 out of 10 times speed brakes are required if atc had asked to reduce speed.
the only catch in this is the cabin v/s which can sometimes go really high and which is a reason why some people prefer managed descent to avoid that.
as for fuel saving as long as you are not having level segments you should be good. incidentally if at cruising level your fuel flow is 1200 kg/hr/eng, at 1500 agl your fuel flow will not be lesser than 1100/kg/hr/eng. thats like a difference of 3-4 kgs a min.
in the end fly the descent that makes you the happiest, and be stabilised by 1000 ft. thats all that matters.