One of the strengths of Upgrade is the cycling ability that comes with it being a self replacing non-terminal. In my mind, the cost difference between Upgrade and Remake is justified by the ability to play them without slowing down your deck progression, since Upgrade replaces itself and gives an extra action. Remake is a powerful trasher, but sometimes buying and using it can potentially fatally slow down the tempo of deck progression: a turn buying or using Remake will often yield no other benefit.
Upgrade rarely is an outright bad buy in the first four rounds, as there are not too many Kingdoms that will be hurt by the easy trashing of Coppers or Estates. Chapel or Apothecary scenarios might not benefit from Upgrade and on scary BMU boards Upgrade may be a deadly distraction, but on the whole it is a very strong card, especially if the Kingdom has $3 non-terminals. Think Menagerie, although even Wishing Wells and Villages can be great targets for Upgraded Estates.
It is certainly possible to go to far and buy too many Upgrades. There are times, however, when it makes sense to buy multiple copies going even into the mid-game. Remember, extra Upgrades can be easily converted into Gold in turn 7-10 to make a strong push into the VP buying stage of the game. This is board dependent. In non-treasure setups it can be a very strong play to buy multiple Upgrades in order to quickly cycle and kill the starting cards. Multiple Upgrades can also help against Cursing attacks, if you can stay on top of it and not let your opponent get too far ahead.
In <a href=http://dominion.isotropic.org/gamelog/201109/09/game-20110909-043915-71a66b40.html>this game</a>, the winning difference is the early inclusion of multiple Upgrades. I was able to quickly trash the starting cards, gain multiple Peddlers and Upgrade those Peddlers into Platinum for an easy win against a very good player.
In <a href=http://councilroom.com/game?game_id=game-20110528-120142-1060940b.html>this game</a>, I used Upgrade to accelerate Alchemist and used Expand to turn the Upgrades into Provinces for a faster close against Saboteur attacks.
In <a href=http://councilroom.com/game?game_id=game-20110725-022058-752cf29a.html>this game</a>, I was able to build a deck made up entirely of non-terminals and Goons, three-pile ending in a win in 15 turns with no VP buys.
In <a href=http://councilroom.com/game?game_id=game-20110628-163405-9b8610ef.html>this game</a>, I way over-gained Upgrades and got spanked by an opponent who never bothered trashing his starting hand, and instead used his Coppers in conjunction with Laboratories and Banks to thoroughly whip me.