Upgrade is quite strong by itself for two reasons:
You can use it to trash 0-cost cards that you no longer want in your deck (coppers and curses).
Trashing curses is obvious, but regarding copper: do not underestimate the power of having a really small deck!
You can use it to turn your Estates into Silver (always a good choice) or, even better, into a highly contested 3-cost card in a 3 or 4 player game (think Village in a pool where it is the only action-gaining card).
These reasons are enough for Upgrade to be something you want to have in your deck, and not only in one copy.
There are also, in my experience, two factors stemming from the kingdom card pool that "modify" Upgrade's usefulness: If the pool is very strongly combo-oriented (lots of cards with +actions and +cards plus cards with +gold and/or +buy), Upgrade may be too slow.
For example, in a pool with Festival/Smithy/Village/Nobles/Throne Room I don't think you 'd have the time to do enough with Upgrade before combos start exploding left and right.
If the pool has no +buy actions , Upgrade is stronger.
It's the only way you will have to gain a second card in your turn.
In this case, lots of Upgrades (3-4) can serve to turn your initial turn buys (2-4 cost) to 5-6 cost cards.
And finally, don't forget that if you have too many Upgrades you can always use one to turn another into a Gold.
boardgames.stackexchange.com/users/611/Jon
You can use Upgrade to gradually improve your deck while also carrying out a larger strategy.
Because Upgrade is non-terminal (that +1 Action)
you can turn your Estates into Silver or middling 4-cost cards into more powerful 5-cost cards while still playing your other Actions.
It is tricky, I won't deny that, but it can have some value.
I don't see it as the core of your strategy, though.
boardgames.stackexchange.com/users/32/lilserf
In addition to the +1 Card and +1 Action, Upgrade is one of only two "gain a card costing X more"
(note: I'm ignoring Forge here) cards that has the word exactly on it (the other is Cornucopia's Remake).
Meaning that if you Upgrade a Copper or Curse, you get nothing back (assuming you didn't play a Bridge before it).
With Remodel and Expand, you must gain another card costing up to 2 or 3 more, even if the cost is lower than that of the card you trashed.
boardgames.stackexchange.com/users/87/Powerlord
There are two key points here: Your deck starts out bad.
Many cards are much better in the early game than later.
You can use upgrade to solve both problems.
Getting rid of your starting copper (and likely estates) will improve your deck.
So will trashing that first-turn buy
that's just too weak to get you to provinces later.
It's a non-terminal action, so it gives you a way to trash these cards without wasting turns, as you tend to do with something like chapel or steward.
And do remember: it's an upgrade .
You're not just removing a weak link, you're replacing it with something to strengthen your deck.
There are some great special cases, too, like upgrading four-cost actions to duchies in the end game, and trashing curses.
boardgames.stackexchange.com/users/409/Cascabel
You can buy many Upgrade as early as possible in the game.
This is a really good starting strategy.
By many, I mean as much as 5 or 6 cards.
It will clean your deck so quickly from all the Copper and Estate , you will be able to upgrade your Upgrade into Gold and reach the Province much faster than you can imagine.
And don't forget to upgrade your Estate to Great Hall (instead of Silver ) if both are on the table!
boardgames.stackexchange.com/users/125/Didier Trosset
Upgrade allows you to thin your deck with a non terminal action.
Being able to get rid of coppers and curses is almost always helpful.
You an also trade out poorly performing or obsolete cards for better cards.
Heck, upgrade an Upgrade to a gold :)
I love being able to thin my deck and will almost always grab 1-2 of these if there are no better deck thinning options available.
boardgames.stackexchange.com/users/40/Pat Ludwig
The biggest advantage of Upgrade is it helps to clean the junk (coppers, estates, and curses especially) out of your deck fast.
And then, when you don't need them, turn the upgrades into gold or other 6 cost cards.
Here as an example of game of how to use them effectively: http://dominion.isotropic.org/gamelog/201106/18/game-20110618-132616-55486cf0.html
boardgames.stackexchange.com/users/1445/user1445
In what situations is Upgrade (Intrigue) useful? I have never seen anyone buy Upgrade in the games I've played where it was available. Whenever I play with Upgrade and Swindler, I always give Upgrade to my opponents in exchange for their 5-cost cards. They never know what to do with it... What are we doing wrong? How can we make use of this card?
Proper use of Dominion card Upgrade
boardgames.stackexchange.com
<dominion><dominion-intrigue>
boardgames.stackexchange.com/questions/1615
boardgames.stackexchange.com/users/369/Marcin
You can use it to gradually improve your game. Upgrade helps to clean the junk out of your deck. You can use it to trash cards you no longer want. You can then turn the Upgrades into gold cards. The biggest advantage of Upgrade is it helps to clean out cards you no longer want, such as coppers, estates, and curses out of your deck. You can also upgrade four-cost actions to duchies at the end game. You can use Upgrade to gradually improve your deck while also carrying out a larger strategy.  You can use Upgrade to turn your Estates into Silver or Great Hall or 4-cost cards into more powerful 5-cost cards. Upgrade enables you to think your deck and trade out poorly performing or obsolete cards for better cards.  
And then, when you don't need them, you can turn the upgrades into gold or other 6 cost cards.
The biggest advantage of Upgrade is it helps to clean out cards you no longer want, such as coppers, estates, and curses out of your deck. You can also upgrade four-cost actions to duchies at the end game. You can use Upgrade to gradually improve your deck while also carrying out a larger strategy.   You can use Upgrade to turn your Estates into Silver or Great Hall or 4-cost cards into more powerful 5-cost cards. Upgrade enables you to think your deck and trade out poorly performing or obsolete cards for better cards.   And then, when you don't need them, you can turn the upgrades into gold or other 6 cost cards.