The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
The ability to get the full package of draw (that is, increasing your handsize while maintaining your ability to play more action cards) from a single card is very powerful, and any card which allows it without having to jump through too many hoops deserves significant consideration.
Seer gives you this ability, with the only stipulation that the cards need to cost between 2 and 4. In general, if you are drawing less than one extra card off Seer, it’s mediocre; at around 1 card, it’s pretty decent. But it’s when you start getting multiple extra cards from a single Seer that it starts to get particularly powerful. Will you get to that point? That depends a lot on how you build your deck – how well can you get rid of your coppers? Are you getting junked? Do you want cards that cost 2-4, or does most of your ideal deck cost 5+? Of particular note is that, barring cost-reduction, Seer doesn’t cost 2-4, meaning that you can’t just build a stack of only Seers and have everything draw beautifully. Overall, though, the range is really wide on how good it will be, and you need to look at every case on its own merits; thankfully, this is not so hard to do, so long as you roughly know your plan and your deck’s composition.
One note about the action phase when playing multiple Seers – try to give each Seer the maximum possible search space to hit draw-able cards; this is done by not having multiple wrong-costed cards on top of your deck when you play it, playing other draw cards (most often cantrips) in between Seers when necessary.