The original (old, large can-style) in my '88 lasted about 175,000 miles. The first replacement (new, small style) was grossly inaccurate right out of the box. Dealer admitted this is a problem and replaced it. Second replacement was also bad. Third replacement was reasonable accurate and has been in for about 60,000 miles, but in the last few days I've noticed it says my idle oil pressure is 45 psi (highly unlikely), so I assume it's on the way out.
I doubt an after-market unit will be any better, but at least it might be cheaper.