Exploration is obviously nice for maps where you end up with lots of coastal cities (small continents have nice maps), its just that there are several other good option in other trees. I was thinking few times about taking some points there, but always commerce or rationalism comes ahead, sometimes I just want Patronage or Aesthetics. Later on Ideologies.. Its decent tree, anyways some thoughts:
Navigation school - A Great Admiral appears. +2 Movement for all Great Admirals. Great Admirals are earned 25% faster. - I find this mostly a letdown, AI is dumb on the sea anyway and fleet is often spread around to benefit from Admiral. Its nice to have Admiral bonus when bombing cities, but you could just take one more turn to bomb the city and live fine anyway.
Naval tradition +1 Happiness for each Harbor, Seaport, or Lighthouse. - Some happy faces are nice, but many trees have better happy bonuses.
Merchant navy (Civ5) Merchant navy +1 Gold for each Harbor, Seaport, or Lighthouse. - This is also a bit weak, like +2.x gold per city, and these building cost maintenance and hammers and are not useful everywhere.
Rest of the policies are very good, opener and finisher especially. I think I might try playing Portugal with Liberty and Exploration for culture win.