I think the stats show that almost all men after prostatectomy who can still get erections have less effective erections, which probably backs up Dave's point that some nerves get damaged even in claimed full nerve sparing. Often this doesn't prevent intercourse if you go from rock solid grade 4 erections down to still usable grade 3 erections, although there are circumstances where it might, e.g. if you have anal intercourse where a more solid erection is required than for vaginal intercourse. Known partial nerve sparing is going to make this a little worse, and it's going to take longer to recover to the point of having usable erections. Some men find this still improving years after prostatectomy (improvements after 3 years are probably due to nerves regrowing, which can happen in a few cases).
However, there are some simple things you can do to improve erections if you're getting them but they aren't as strong as you'd wish. The PDE5 inhibitor drugs (Viagra, Cialis, etc) have the effect of amplifying the nerve signals arriving at the penis, and can often make up for fewer nerves. A cock ring (not a pump constriction ring, but something more gentle such as a Durex Pleasure Ring) can also do this by holding more blood in the penis.