Proper use of Dominion card Upgrade
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.